Cancellation of Agreement to sell

Sir, We have entered into an agreement to sell on 9-Jan-19, for sale of 20 Acre agriculture land in my village, for and amount Rs. 11,02,00,000/-( Eleven crore two lakhs), i.e 55,10,000/- per acre. Where the party paid only 5,00,000/- as advance towards agreement ( 3,00,000 in cash, 2,00,000 in cheque- not yet deposited), and remaining amount to be paid with nine months. Now that my family members are not agreeing to sell the property, and it is only that all we have for future of my children. They are not agreeing to sign on any agreement of registration, now I want to cancel the agreement to sell. On 2-2-19 ,we received a notice from buyer, stating that further amount of 25,00,000/- should be received and enter into further agreement to sell, if not he will file a cheating case on us. What reply should be given to notice. About agreement: 1. Agreement entered in Rs.50/- bond.( Whether that bond is worthy for Eleven cr agreement) 2. As we sellers of property are 2 brothers, and only one brother name is printed twice in agreement ( i.e in place of my name also), but I also signed the agreement. 3. Terms and conditions are as follows: A.Should register property after receiving total sale consideration. B.Get documents arranged. C.Vendor declare that there is no other person have any kind of right, interest( but my children have the right, and they are not agreeing to sell) D. Vendor has no existing agreement. E. Vendor handed over the possession to vendee on agreement day F. Vendor hereby assured and agreed to co operate with vendee in getting the layout approved and all types of other developmental activities and to sign wherever necessity arises. Please give the advice, how we should proceed further to cancel the agreement to sell by returning the advance amount.