Agreement to sell

I bought a flat for 15 lakh in 2015, July when the market value of the flat was approx. 40 lakh, but owing to a prior understanding with owner, the flat was agreed to be sold to be at 15 lakh vide an agreement to sell. Now a change of mind has occurred with owner and he wishes o extort more money from me despite a written agreement to sell. According to agreement I was to pay 15 lakh total through bank account (full and final) which I have already done 6 months back but the owner has not yet agreed for sale deed/mutual transfer (as the owner does not have a conveyance deed as yet).What remedies do I have as no time was stipulated in agreement for sale deed.