How does cancellation/de-registration of agreement to sell happens

An Agreement To Sell executed/registered after making 25% part payment for allotment money against the purchase of flat under construction in Godrej Garden City, SG Highway,Jagatpur, Ahmdabad in January, 2016.Since I have been selling the said flat to the third party, the same is required to be cancelled/de-registered as the rest 75% will be made by the new buyer on receipt of the notice of possession after few months. What is the stamp duty, if any and the cancellation/de-registration charges etc. to be paid in respect thereof? Earlier the stamp duty of Rs.100/- and registration charges of Rs.445/- + 20/- was paid to execute the said ATS in the Sub-Registrar office of SRO, Ahmdabad-8. What should be the real amount?