Sale agreement

I had executed a sale agreement for my plot in Delhi. The agreement was signed on 20 Feb 2014, and the last date of full payment and registration was 19 Dec 2014. The second part stepped back. It was mentioned in the agreement that in case the second party fails to pay the remaining amount (i.e., apart from byana) by 19/12/2014 the agreement shall be automatically cancelled and that the second party will have no claim on the amount given as byana. I did not serve a legal notice to the second party after the date passed. The second part of the story is that I again executed a sale agreement this Feb which expired on 17 June. The second party made the byana payment Rs 5 lacs in cash and 15 lacs through cheque.However, he stopped the payment by cheque. But I did not serve any notice to him. I informed this to the mediator. He told me that due to fire in his factory he can't buy the land. But yesterday he served me a notice stating that he was cheated by me as I had sold this land to someone else (giving reference of my first sale agreement). He has also stated that despite his continuous insistence I did not take NOC from the concerned department. Now he is demanding double the amount of byana he has given me otherwise filing a lawsuit under section 420 of IPC. Please advice me what to do as I only have a few days to answer.