Part-payment from buyer after sale deed.

Hi Team, We are selling a property in Bilaspur, C.G for Rs.40L. Transaction details are as follows - 3L - Advance. 32L - Home Loan. 5L - Post dated cheques. The buyer is a close associate and is requesting for post registraion payment of Rs 5L as he's unable to arrange for another loan at present. He is insisting that he would pay the remaining 5 lakhs from his Renovation Loan approved by LIC. This trasaction cannot be delayed further due to personal reasons and he is the only buyer we have. Hence we want to get this deal done soon enough but with proper measures. How should we deal in this situation so that we are protected in terms of any post dated cheque failure or any such payment failure after registration? Kindly provide measures and legal document suggestions for the same. Thanks, Kiran