Judgment :- J.B. Koshy, J. These two writ appeals are filed against the very same judgment in O.P. No. 4196 of 1989. They were heard and disposed of together. 2. The above O.P. was filed by a public limited company engaged in the manufacture and sale of Indian made Foreign liquor. Third respondent in the O.P. Kerala State Beverage Corporation thereinafter referred to as "the Corporation") is incorporated by the Government of Kerala with a view to constitute as the sole agent in the State for distribution of foreign liquor with effect from 1.4.1984. The Corporation was issued licence under the Kerala Abkari Act and the Rules authorising it to store in bond, foreign liquor. As can be seen from Ext. P2, the Corporation can purchase liquor without payment of excise duty and can be stored in the bonded warehouse on condition that they are removed for distribution from the bonded warehouse only on payment of excise duty payable and on obtaining a pass from the Excise Department to that effect. In other words, the Corporation was authorised to keep non-duty paid foreign liquor manufactured in the distilleries in Kerala in the bonded warehouse and to conduct sale of liquor to wholesale shops after realisation of excise duty and other dues. From 1.4.1984 itself petitioner company started supply of Indian made Foreign liquor to the third respondent Corporation and it is stated that both the petitioner company and the Corporation were under the belief that no sales tax will be payable on the excise duty element as goods were removed from the petitioner company which is manufacturing IMFL without payment of excise duty and Corporation collects excise duty when it supplies to other agencies. It is not disputed that under the provisions of the Kerala General Sales tax Act, 1963 the liability to pay sales tax on the turn over of Indian made Foreign liquor was on the petitioner, the petitioner being the first seller within the State and sale tax is being collected from the customers. 3. In McDowell & Co. Ltd. v. Commercial Tax Officer (39 STC 151) it was held that excise duty payable by the buyers of liquor and not included in the sale bills issued either by the manufacturer cannot form part of the turn over and were not liable to sales-tax. 3. In McDowell & Co. Ltd. v. Commercial Tax Officer (39 STC 151) it was held that excise duty payable by the buyers of liquor and not included in the sale bills issued either by the manufacturer cannot form part of the turn over and were not liable to sales-tax. The above legal position was changed in May, 1983 when in McDowell & Co. Ltd. v. Commercial Tax Officer (59 STC 277) the Supreme Court has clearly held that the liability to pay excise duty is on the manufacturer and notwithstanding the payment of the said amount by the purchasers the element of excise duty forms part of the turn over of the manufacturer and sales-tax is liable to be levied on the element of excise duty. In view of the above decision, petitioner company demanded sales-tax on the element of excise duty also from the Corporation as, ultimately, they will be liable to pay sales-tax on the turn over of Indian made Foreign liquor and the petitioner company wrote to the third respondent Corporation regarding these matters and requested the third respondent to move the Government to make first point sale under bond without sales-tax so that this uncertainty and difficulty could be removed and the Corporation itself can pay the sales-tax directly. The matter was further discussed by the petitioner and the third respondent and on the basis of the detailed discussions Ext. P5 undertaking dated 31.5.1984 was given by the Corporation. The above undertaking signed by the Managing Director of the Corporation states as follows: "With reference to the various purchase orders placed/and being placed with you for the supply of IMFL, the rates are exclusive of excise duty since the same is payable by us at the time of release from our bonds. In case at the time of your sales-tax assessment, the assessing authority assesses the excise duty also to tax we undertake to reimburse to you sales tax and additional sales-tax, that becomes payable by you on the element of excise duty". In view of the above undertaking, petitioner company did not collect sales tax and additional sales tax that becomes payable on the element of excise duty as the Corporation undertook that if the assessing authority assesses the excise duty also to tax that will be reimbursed by the Corporation. In view of the above undertaking, petitioner company did not collect sales tax and additional sales tax that becomes payable on the element of excise duty as the Corporation undertook that if the assessing authority assesses the excise duty also to tax that will be reimbursed by the Corporation. Following the Supreme Court decision reported in 59 STC 177 (supra) pre-assessment notices were served on the assessee and sales tax was also adjusted on this turn over by the Sales-tax Department from the refunds due to the petitioner company. Petitioner requested for Sales-tax and additional sales tax on the excise duty turn over from the Corporation as originally sales-tax was collected from the Corporation only on the turn over exclusive to excise duty element in view of the undertaking. By Ext. P7 letter dated 18.1.1985 the above undertaking was withdrawn. It is stated as follows: "In our letter No. FM/84-85 dated 31st May, 1984, we undertook to reimburse the sales-tax and additional sales-tax on the element of excise duty if the same become payable by you at the time of sales-tax assessment. We wish to inform you that we are withdrawing the undertaking given in the above letter and will not be able to reimburse to you any amount on the above account." Therefore, by Ext. P8 letter dated 7.3.1985, the Corporation informed the petitioner company that the withdrawal of undertaking will be with retrospective effect and will be applicable for all supplies made by the petitioner. By Ext. P9 dated 8.7.1985 the Corporation was informed by the petitioner that the undertaking for reimbursement was given after detailed discussions and petitioner company is not accepting the withdrawal letter and requested for making reimbursement of the purchase even though they are contesting the matter with the Sales-tax authorities. By Ext. P10 letter dated 11.7.1985 the Corporation wrote back stating that for withdrawal with retrospective effect acceptance is not required. The letter states as follows: "The offer to reimburse the amount of sales-tax and additional sales-tax on excise duty on liquor supplied from your Kerala unit was purely voluntary and made on the bonafide belief that there would be no such liability as per the law in force in the State. Its withdrawal with retrospective effect does not therefore, require your acceptance. Its withdrawal with retrospective effect does not therefore, require your acceptance. If the assessing authority has proposed to add exercise duty in your turn - over, it is for you to explain to him the correct legal position. In case he has pointed out our undertaking to reimburse the amount, you are supposed to show to him that it has been revoked by us." Thereafter, several requests were made by the petitioner company to the Corporation. Some of the requests were produced as Exts. like Exts. P9, P10A etc. and debit notes were also issued. 4. Final sales tax assessment was made by Exts. P11 and P12 for the years 1983-84 and 1984-85 on 12.5.1986 wherein sales-tax was assessed on the petitioner company including the turnover of excise duty element also and demands were issued by Exts. P13 and P14 in October, 1986. On receiving the same, enclosing the copy of assessment orders, petitioners wrote to the Corporation for reimbursement of sales-tax and additional sales-tax as per Ext. P5 undertaking. Several discussions took place and several letters like Exts. P16, P17, P18 etc were sent. Meanwhile, the matter was also contested before the statutory authority under the Sales Tax Act. In 1987, appeal filed before the Appellate Assistant Commissioner was also dismissed and appeals were filed before the Tribunal. Petitioner company also wrote to the Minister, Government of Kerala regarding this matter with copy to the Corporation. Exts. P20, P21, P 22 etc show that the Corporation which is the wholesale distribution of IMFL in the State of Kerala has collected sales-tax, additional sales-tax etc. from their purchasers. Even though they have collected sales-tax from their purchasers, it was not paid to the first seller for onward payment to the Government despite their undertaking. As instructed by the Corporation appeal was also filed before the Tribunal and an impleading petition was also filed in the appeal proceedings for impleading the Corporation as a party as the Corporation has collected the amount and undertaking was also given to reimburse the sales-tax amount. The above impleading petition was rejected by order dated 30.3.1989, Ext. P27. Since all the efforts to get the amount on the basis of Ext. P5 undertaking were failed, petitioner company approached this Court. Apart from the prayer for quashing Ext. The above impleading petition was rejected by order dated 30.3.1989, Ext. P27. Since all the efforts to get the amount on the basis of Ext. P5 undertaking were failed, petitioner company approached this Court. Apart from the prayer for quashing Ext. P27 order rejecting the impleading petition the following two prayers were also made: "C. Issue a writ of mandamus or any other appropriate writ, order or direction to the 3rd respondent to honour the undertaking dated 31.5.1984 (Ext. P5) and to reimburse the amounts due to the petitioner with interest at the rate of 18% per annum from the date on which the amount became due; D. Call for the records leading to the issue of Ext. P7 communication dated 18.1.1985 unilaterally withdrawing the undertaking (Ext. P5) and quash the same by the issue of a writ of certiorari". It was also contended by the petitioner that Exts. P7 and P8 withdrawal are unreasonable and arbitrary. Ext. P15 is a statement showing the amount of sales-tax etc. due to the element of excise duty for the supplies made to the third respondent. This statement is not disputed. 5. Considering the pleadings raised by the parties, the learned single judge held as follows: "As regards the retrospective withdrawal of the undertaking contained in Ext. P5,1 am of the view that the said attempted withdrawal retrospectively cannot operate so as to disable the petitioner from relying on the representation contained in Ext. P5 and in arranging its affairs accordingly. At best the withdrawal could come into operation on the date of Ext. P7 when the 3rd respondent Corporation informed the petitioner that it was withdrawing the undertaking given under Ext. P5. It is necessary at this stage to consider the circumstance under which Ext. P5 was written. It is the specific case of the petitioner that in the light of the second McDowell's case (59 STC 277) controversy and doubt arose regarding the exigibility to tax, additional tax and surcharge on the excise duty element and such a doubt arose by about the middle of May, 1984. It has to be noted that this contention of the petitioner has not been denied by the Corporation in its counter affidavit. It has to be noted that this contention of the petitioner has not been denied by the Corporation in its counter affidavit. It is in the light of this doubt that arose during the middle of May, 1984 that discussions were held between the parties and the 3rd respondent Corporation gave the undertaking contained in Ext. P5 and communicated the same to the petitioner. It was a conscious undertaking. It is acting on this undertaking that the petitioner did not collect the sales-tax, additional sales tax and surcharge on the excise duty element from the third respondent for the period subsequent to Ext. P5. In the absence of a case that the petitioner has already collected sales-tax etc. due on the excise duty element from the 3rd respondent, I am of the view that the petitioner can invoke the doctrine of estoppel against the 3rd respondent in respect of the sales tax etc. not collected subsequent to the date of Ext. P5 and which it has been found liable to pay by the orders of assessment by the authorities under the Kerala General Sales Tax Act. B ut the said undertaking can enure only till the date of Ext. P7 when the said undertaking or representation was clearly withdrawn by the 3rd respondent. I therefore, hold that in respect of the liquor supplied during the period between the dates of Exts P5 and P7 the petitioner is entitled to rely on Ext. P5 to seek recovery of the amount of sales tax, additional sales-tax and surcharge due on the excise duty element from the 3rd respondent Corporation." The learned single judge also noticed that the 3rd respondent had already collected sales-tax, additional sales-tax etc. on the element of excise duty from its customers, and, therefore, it had the obligation to pass on the same to the petitioner. Therefore, the learned single judge held that: "This is therefore, a case where the 3rd respondent has not only not paid the tax payable by it to the petitioner which the petitioner was bound to make over to the State, but has also retained the amount collected. The undertaking contained in Ext. P5 relied on by the petitioner has to be appreciated in the context of the facts The undertaking Ext. P5 is clearly to the effect that if the petitioner was assessed to sales-tax etc. The undertaking contained in Ext. P5 relied on by the petitioner has to be appreciated in the context of the facts The undertaking Ext. P5 is clearly to the effect that if the petitioner was assessed to sales-tax etc. on the element of excise duty the 3rd respondent will reimburse the petitioner in respect of that amount. The petitioner in fact has been assessed and the recovery itself has been effected by the first respondent against the petitioner. I therefore, held that the 3rd respondent is bound to reimburse the petitioner the amounts due towards sales tax, additional sales-tax and surcharge on the excise duty element for the period between 31.5.1984 and 18.1.1985". The learned single judge also found that after Ext. P7 withdrawal no estoppel will apply and 3rd respondent Corporation has no obligation to pay sales-tax and surcharge after Ext. P7 withdrawal. The learned single judge further held as follows: "I therefore, find that by Ext. P7 there has been a valid withdrawal of the undertaking by the 3rd respondent and therefore, there would be no obligation on the 3rd respondent in respect of the sales-tax, additional sales-tax and surcharge on the element of excise duty in respect of the period subsequent to Ext. P7. The learned judge was also of the view that there was unjust enrichment as the Corporation had already collected tax from their customers and, therefore, there is an obligation to pay the same. The learned Single Judge further held as follows: "If the amounts were permitted to be kept by the 3rd respondent in the circumstances of this case, it would really be a case of unjust enrichment This amount is therefore, something which is not due to the Corporation. If the Corporation were not to be pinned down to the promise made by it in Ext. P5 and were to be permitted to keep this amount, it will really amount to a case of unjust enrichment and I am of the view that this Court cannot refuse to act when the attempt of the 3rd respondent is to keep the amount for itself undeservedly". In view of the above finding the 3rd respondent Corporation is estopped from denying the obligation to pay over the amount in question of sales-tax, and additional sales-tax on the element of excise duty in terms of Ext. P5 till Ext. P7 was issued. In view of the above finding the 3rd respondent Corporation is estopped from denying the obligation to pay over the amount in question of sales-tax, and additional sales-tax on the element of excise duty in terms of Ext. P5 till Ext. P7 was issued. The question regarding the correctness of the Tribunal's order was not decided as the O.P. was allowed and directed the "3rd respondent to reimburse the amounts due to the petitioner as sales tax, additional sales tax and surcharge on the excise duty element in respect of the Indian made Foreign liquor supplied by the petitioner to the 3rd respondent for the period from 31.5.1984 to 18.1.1985 with interest thereon at 12% per annum from the respective dates of supply". 6. The 3rd respondent Corporation filed Writ Appeal No. 750/94 raising the following grounds. (i) As there is no violation of the fundamental rights writ petition under Art.226 of the Constitution will not lie even though the 3rd respondent is a State coming within the purview of Art.12 of the Constitution; (ii) It was contended that the matter arises on contractual matter and it cannot be a subject matter of judicial review under Art 226 of the Constitution and proper remedy in such case will be to approach the civil court; (iii) There is undue delay and therefore, the O.P. is barred by limitation; (iv) Principles of promissory estoppel cannot be applied on the facts of this case; (v) The learned single judge ordered not only payment of sales tax and additional sales tax but also surcharge for which there was no undertaking; (vi) Neither in the undertaking nor in the subsequent demands from the company, there was no request for interest and interest was claimed for the first time in the O.P. and therefore, grant of interest prior to the period of filing the O.P. is illegal. 7. Petitioner company also filed Writ Appeal, No. 1268/94 claiming interest at the rate of 18% as the learned judge has granted only 12% interest. It is also contended that Ext. P7 itself is bad on the principle of estoppel and the undertaking given after discussions cannot be withdrawn unilaterally and on the very same principle refund should be granted also for the period from 18.1.1985 to 12.3.1985. 8. It is not disputed that 3rd respondent Corporation is a State coming within the meaning of Art.12 of the Constitution. P7 itself is bad on the principle of estoppel and the undertaking given after discussions cannot be withdrawn unilaterally and on the very same principle refund should be granted also for the period from 18.1.1985 to 12.3.1985. 8. It is not disputed that 3rd respondent Corporation is a State coming within the meaning of Art.12 of the Constitution. But, the contention raised is that since none of the fundamental rights of the petitioner is affected, remedy of Art.226 will not apply against the instrumentality of the State. It is the contention of the petitioner that Ext. P7 withdrawal is unreasonable and arbitrary and if it is unreasonable and arbitrary, there is violation of Art.14 of the Constitution. The point that there is no violation of the fundamental rights and, therefore, writ will not lie was not seem to have raised or argued before the learned Single Judge. Apart from the above, if withdrawal of the undertaking is unreasonable and arbitrary, there is violation of Art.14. What is found by the learned single judge is that petitioner was bound to collect sales tax on the turn over of excise duty also and since there was some doubts on the above point after discussion, Ext. P5 undertaking was given and if the petitioner is assessed the sales tax and additional sales tax will be reimbursed to the petitioner. In fact, the Corporation has also collected the amount from the customers on the basis of the promise the petitioner acted upon and incurred liabilities and the subsequent withdrawal with retrospective effect is unreasonable and arbitrary. It is the case of the petitioner that writ petition will not lie against the company in purely commercial matters. Art.14 of the Constitution of India reads as follows: "The State shall not deny to any person equality before the law or the equal protection of the laws within the territory of India". It was held by the Supreme Court repeatedly that the word 'State' in this article is to be understood in the sense used in Art.12 (Budhan v. State of Bihar: AIR 1955 SC 191; Lakshmanarao v. Judl. Magistrate :191 SC 186 and Ramana v. The International Airport Authority of India : AIR 1979 SC 1628). It was held by the Supreme Court repeatedly that the word 'State' in this article is to be understood in the sense used in Art.12 (Budhan v. State of Bihar: AIR 1955 SC 191; Lakshmanarao v. Judl. Magistrate :191 SC 186 and Ramana v. The International Airport Authority of India : AIR 1979 SC 1628). It was held that the word 'State' includes all instrumentalities or agencies of the Government whether they are departments of the Government or not, whether a statutory corporation or other private body. In Dwarkadas v. Bombay P.T. ((1989) 3 SCC 293) it was clearly held by the Supreme Court that even contractual dealings tainted by arbitrariness is violative of Art.14 of the Constitution. On the facts, it can be seen that Ext. P5 is a clear and unambiguous undertaking to reimburse to the petitioner sales-tax and additional sales-tax on the excise duty turn over if petitioner was assessed to tax. Of course, by Ext. P7 the above undertaking was withdrawn. It is not mentioned even in Ext. P7 that it was withdrawn retrospectively. But, subsequently, in Ext. P8 it is stated that it is withdrawn retrospectively. On the facts of the case, it is clear that withdrawal of the undertaking retrospectively is clearly unreasonable and arbitrary and there is violation of Art.14 of the Constitution of India and we are of the view that withdrawal of Ext. P5 undertaking by Ext. P7 retrospectively is arbitrary, unconscionable and unreasonable and writ will lie. 10. In Food Corporation of India v. Kamdhenu Cattle Feed Industries (1993) 1 SCC 71) it was held as follows: "In contractual sphere as in all other State actions, the State and all its instrumentalities have to conform to Art.14 of the Constitution of which non-arbitrariness is a significant facet. There is no unfettered discretion in public law. A public authority possesses powers only to use them for public good. This imposes the duty to act fairly and to adopt a procedure which is" fairplay inaction'." The Supreme Court has, in various cases, accepted the principle laid down in Associated Provincial Picture Houses Ltd. v. Wednesbury Corporation ((1947) 2 ALL ER 680) which is called irrationality, namely, Wednesbury unreasonableness as a ground for judicial review - See G.B. Mahajan v. Jalgaon Municipal Council ((1991) 3 SCC 91). Of course, it is true that if there is no malafides bias or arbitrariness, this Court will not interfere in contractual matters. In Tata Cellular v. Union of India reported in ((1994) 6 SCC 651) after analysing the entire English and Indian case laws it was held as follows: "The Government must have freedom of contract. In other words, a fair play in the joints is a necessary concomitant for an administrative body functioning in an administrative sphere or quasi-administrative sphere. However, the decision must not only be tested by the application of Wednesbury principle of reasonableness (including its other facts pointed out above) but must be free from arbitrariness not affected by bias or actuated by malafides." Here, on the basis of the undertaking, petitioner company supplied IMFL to the Corporation and the Corporation, in turn, collected sales-tax and additional sales tax also on the excise duty turn over. But, when assessment was made and petitioner requested for reimbursement the undertaking was withdrawn. Even though the Corporation is entitled to withdraw its promise it can only be done prospectively and retrospective withdrawal making the other party to suffer without any reasonableness is arbitrary and facts as correctly found by the learned single judge warrants interference by this Court under Art.226 of the Constitution. The learned Single Judge correctly followed the principle laid down in Gujarat State Financial Corporation v. M/s. Lotus Hotels Pvt. Ltd. (AIR 1983 SC 848) where the Supreme Court held as follows: "It is too late in the day to contend that the instrumentality of the State which would be the other authority under Art.12 of the Constitution can commit breach of a solemn undertaking on which other side has acted and then contend that the parties suffering by the breach of contract may use for damages but cannot compel specific performance of the contract." It is the statutory obligation of the petitioner to collect excise duty when it sell the material to the Corporation. But, only on the basis of undertaking it has not collected the same. But, only on the basis of undertaking it has not collected the same. Having collected the amount from its customers the Corporation should have passed on the amount of sales-tax collected to the petitioner when assessment was made in pursuance of the undertaking and withdrawal of the above undertaking retrospectively is arbitrary and the learned single judge after considering the facts of the case clearly found that even without going for a civil case, petitioner can approach this Court against the statutory Corporation for its arbitrary and unreasonable action. 11. The next question is regarding the application of the principles of promissory estoppel in this case. Promissory estoppel is defined in Black's Law Dictionary as: "that which arises when there is a promise which promissory should reasonably expect to induce action or forbearance of a definite and substantial character on the part of promisee, and which does not induce such action or forbearance, and such promise is binding if injustice can be avoided only by enforcement of promise." The principle as reiterated by Lord Denning in Central London Property Trust Ltd. v. High Trees House Ltd. (1947 KB 130) is that "A promise intended to be binding, intended to be acted upon, and in fact acted upon is binding " In Pawan Alloys & Casting Pvt. Ltd. v. U.P. State Electricity Board and Ors.. ((1997) 7 SCC 251) it was held as follows: "Suffice it to say at this stage that if a statutory authority or an executive authority of the State functioning on behalf of the State in exercise of its legally permissible powers has held out any promise to a party, who relying on the same has changed its position not necessarily to its detriment, and if this promise does not offend any provision of law or does not fetter any legislative or quasi-legislative power inhering in the prom is or, then on the principle of promissory estoppel the promisor can be pinned down to the promise offered by it by way of representation containing such promise for the benefit of the promise", (para 11) In the above case, it was held that if a promise or representation is given by a State or its instrumentality which promise is not against any statute or opposed public policy and on such promise or representation promisee has acted upon and changed its decision to detriment, the State cannot withdraw its promise or representation and cannot resale from the same at least during the period when the promise was there. In that case, Electricity Board by notification under S.49 of the Electricity (Supply) Act, pursuant to the directions issued by the State Government, issued incentive scheme for new industries in its commercial interest to attract more consumers of electricity and concessional rate was promised for three years for new industries. It was held that if new industries were established by spending huge amount, Board would be bound by the principle of promissory estoppel not to resile from its promise or representation before expiry of three years. However, it was also held that if new industries have come after the withdrawal of the notification, within the above three year period but after the withdrawal of the notification, they cannot claim the benefit. In this connection, we also refer to the decision of the Supreme Court in Motilal Padampat Sugar Mills Co. Ltd. v. State of U.P. ((1979) 2 SCC 209; Shrijee Sales Corporation, v. Union of India ((1997) 3 SCC 398); Pournami oil Mills v. State of Kerala ((1987) 1 SCR 654) and Pine Chemicals Ltd. v. Assessing Authority ((1992) 2 SCC 683). 12. In Dr. Ltd. v. State of U.P. ((1979) 2 SCC 209; Shrijee Sales Corporation, v. Union of India ((1997) 3 SCC 398); Pournami oil Mills v. State of Kerala ((1987) 1 SCR 654) and Pine Chemicals Ltd. v. Assessing Authority ((1992) 2 SCC 683). 12. In Dr. Ashok Kumar Maheshwari v. State of U.P. & Ann (1998) 2 SCC 502) it was held as follows: "Doctrine of promissory estoppel' has been evolved by the courts, on the principles of equity, to avoid injustice", (para 8) In Union of India v. Anglo-Afghan Agencies (AIR 1968 SC 718) the Supreme Court held that even though the case may not fall under S.115 of the Evidence Act which enacts the rule of estoppel it would still be open to a party who had acted on a representation made by the Government to claim that the Government should be bound to carry out the promise made by it. In Century Spinning & Manufacturing Co. Ltd. v. Ulhasnagar Municipal Council ((1970) 1 SCC 582); Radhakrishna AgaRWal v. State of Bihar ((1977) 3 SCC 457); and in Union of India v. Godfrey Philips India Ltd. ((1985 4 SCC 369) etc. this principle has been explained and elaborated. On the facts of this case as correctly found by the learned single judge there was a clear promise of reimbursement by the Corporation and on that promise petitioner company was acted upon to its detriment without collecting sales-tax with the hope that as promised in Ext. P5 undertaking if finally assessed Corporation will reimburse the same. But, reimbursement was denied. Withdrawal of the above promise cannot be done retrospectively as the petitioner company had already acted upon and rule of promissory estoppel is correctly applicable in this case. 13. Next contention is regarding delay. Such contention was not seem to have been raised before the learned single judge and His not arising out of the judgment under appeal. Further after withdrawal of the undertaking offered by Ext. P5 dated 31.5.1984 only by Ext. P8 dated 7.3.1985 the petitioner company was informed that the withdrawal of the undertaking was with retrospective effect. Petitioner can claim reimbursement only after assessment is over and assessment was passed only on 12.5.1986. Demand under S.7 was made only in October, 1986 (Exts. P13 and P14). While denying the liability to reimburse sales tax on excise duty, the Corporation in its Ext. Petitioner can claim reimbursement only after assessment is over and assessment was passed only on 12.5.1986. Demand under S.7 was made only in October, 1986 (Exts. P13 and P14). While denying the liability to reimburse sales tax on excise duty, the Corporation in its Ext. P10 letter also advised as follows: "If the sales-tax authorities assess you on the excise duty, you may seek redress from the appropriate appellate authorities or the Government." The assessment was challenged before the appellate authorities and the matter was pending in Sales Tax Appellate Tribunal. The contention in ground 'O' of the appeal memo that since the appeal is pending in Tribunal, liability has not become final, appears to be contrary to the plea of delay. Exts. P16, P17 etc. show that the matter was again under discussion and Ext. P19 representation dated 22.12.1988 was made to the Excise Minister. Only after exhausting all efforts, petitioner company approached this Court. Impleading petition to implead the Corporation was also dismissed on 30.3.198 9 by the Sales Tax Appellate Tribunal and therefore, filing of the writ petition in May, 1989 cannot be held to be belated Especially on the facts of this case when it clearly shows that the Corporation has already collected tax. 14. Next point to be considered is regarding the order of the learned single judge in directing reimbursement of the amount of surcharge. It can be seen that in Ext. P5 undertaking there is no undertaking to reimburse surcharge. Earlier demands made by the petitioner also speaks about reimbursement of Sales-tax and additional sales-tax and not surcharge. S.3(2) of the Kerala Surcharge on Taxes Act prohibits the dealer from collecting any surcharge from the purchaser and the petitioner company could not have collected any amount by way of surcharge from the Corporation and therefore, on the basis of Ext. P5 undertaking reimbursement of surcharge never arises. Therefore, the judgment in so far as it directs reimbursement of surcharge amount is not sustainable in law. 15. With regard to interest awarded from the date of supply, we have gone through various representations filed as Exhibits. In none of the representations, interest was claimed. Petitioner demanded only reimbursement of the amount of sales-tax on the basis of Ext. P5 and penal interest, if any, charged by the department. Penal interest was not charged. 15. With regard to interest awarded from the date of supply, we have gone through various representations filed as Exhibits. In none of the representations, interest was claimed. Petitioner demanded only reimbursement of the amount of sales-tax on the basis of Ext. P5 and penal interest, if any, charged by the department. Penal interest was not charged. Interest was claimed for the first time in the O.P. Even though there was specific undertaking to reimburse the tax if assessment is made assessing sales-tax on the excise duty turn over and assessment was actually made and the amounts were adjusted from the petitioner against the refunds due to it and the Corporation was collecting the amount from the purchasers and was not passing on the amount to the petitioner inspite of the specific undertaking for reimbursement and request, interest can be charged only from the date of demand. Therefore, we are of the view that grant of interest should have been restricted with effect from the date of filing of the original petition. In view of the above, we are of the view that W. A. No. 750/94 filed by the Corporation can be allowed only with regard to the direction to refund surcharge and also grant of interest from the date of supply. We hold that interest shall be payable only from the date of the O.P. 16. With regard to the appeal filed by the company, W. A. No. 1268/94, we see no ground to grant 18% interest as prayed. It is contended by the learned senior counsel for the petitioner company that since there was an undertaking to pay the amount and the Corporation had collected the amount it should have been reimbursed then and there. At the maximum Corporation could have waited for the assessment. Petitioner was holding the amount collected from the purchaser in a fiduciary capacity, interest, commercial interest at 18% should be granted. The above prayer cannot be accepted as no evidence is produced to show that there was such an agreement to commercial interest or it is the purchaser to pay 8% interest in such cases. In the absence of any specific material we hold that 12% interest granted by the learned single judge is reasonable. 17. We agree with the learned Single Judge that even though the Corporation has got right to withdraw the undertaking it can only be done prospectively. In the absence of any specific material we hold that 12% interest granted by the learned single judge is reasonable. 17. We agree with the learned Single Judge that even though the Corporation has got right to withdraw the undertaking it can only be done prospectively. It was for the petitioner to stop supply after Ext. P7 withdrawal. The contention of the petitioner is that Ext. P5 undertaking was given after discussions whereas Exts. P7 and P8 were issued unilaterally and withdrawal cannot be accepted on the principle of estoppel. As rightly held by the learned single judge that principle of estoppel is not applicable in prospectively withdrawing the undertaking, as the Corporation is free to withdraw the undertaking and there is no agreement to the effect that undertaking is for ever. There is no law or circumstance forbidding the Corporation from withdrawing its promise from the date of communication of withdrawal. Petitioner could have arranged its business in such a way after withdrawal of the promise. Therefore, prayer of the petitioner company in the writ appeal that reimbursement should have been granted for the period after the withdrawal of the undertaking cannot be accepted. 18. In the result, Writ Appeal No. 1268 of 1994 filed by the company is dismissed and Writ appeal No. 750 of 1994 filed by the Corporation is partly allowed to the extent of surcharge and granting of interest prior to the date of filing the O.P. Therefore, we affirm the impugned judgment in so far it directs the third respondent Corporations reimburse the amount incurred by the petitioner as sales tax and additional sales-tax on the excise duty element in respect of IMFL supplied by the petitioner company to the third respondent Corporation for the period from 31.5.1984 to 18.1.1985 (the period in between Ext. P5 and Ext. P7) with interest thereon at the rate of 12% per annum from the date of filing of the O.P., that is on 22.5.1989.