vacant land deal

Sir my father had land of 20*80 in chickaballapur he passed away on 8/12/13 I got into agreement to sell n took advance on 28/12/13 as khata was to b on my mothers name n she passed away in may I got khata in July .I informed orally for buyer to buy he did not respond in Dec 2014 I issued notice to him that property can't b sold till now no reply from them now on 23/04/15 they r now asking for agreement fixed money to sell n I asked for current market value as was mentioned in agreement .so now what shd I do how to cancelled agreement legally or any other options for me