Cancellation of 100 rs stamp paper conditional agreement

Thanx in advance.I entered into a sale agreement on 15th july 2016 for sale of land for 1 crore. The agreement was made on a Rs 100 stamp paper and not registered. The agreement had specific clause saying agreement is time bound and expires on 15th oct 2016 if the sale deed is not executed by that date for any reason what so ever. Property was in my father's name and he expierd.Agreement was made by me and my siblings sighned. Buyer made advance of 10 lacs which was mentioned in agreement. Also mentioned that 15 lacs per month has to be paid for two months and remaining 60 lacs should be paid at the time of registration. Now the problem is he paid 10 lacs intially and not paid remaining amount.Now he is asking for 3 months time for registration and we he is threatening that he will go to court and damanding 10 lacs extra for cancellation of agreement if we are not ready for registration. In agreement mentioned that no money will be returned if he fails to pay the amount in 3 months period which is already lapsed but we are ready to give back 10 lac advance amount but he is demanding 20 lacs. What should i do for agreement to be cancelled. what is the legality for this type of conditional agreement which was not registered