How can I withdraw from an agreement for sale of property

have agreed to sell my property for 14th lakz ie for 10 cents with a house..and received a token of advance of 1 lak one month b4 and an agreement is written stating that the buyer should pay the balance within 6 months. But we told them that the other 1lak I should get on or b4 10th December, that which I failed to write in agreement. They accepted my condition and yesterday only they denied my statement. Now there comes another party who could pay 16 lakz ready payment..but former people didn't agrees to withdraw their agreement by repaying their Advance amount.. What should I do on behalf of law.. Also I'm ready to pay them a penalty money for one month. I'm in an urgent need for money.. please tell me the next step which I have to do under law