Property Registered before the full n final payment

Hi, I am Richa from Dadra and Nagar Haveli. My father has a 1bhk flat here which he wanted to sell. A distant relative of mine agreed to buy the same for 13,80,000 rs. The purchaser is working for a private company and wanted to take a housing loan to buy our flat, he wanted alteast 10-12 lacs to be the loan amount for which he convinced my father to make an agreementt of sale on the value of 16lacs. So now he applied for the loan in DHFL and on the basis of his eligibility, 10 lacs has been sanctioned but before the disbursement bank asked us to get the sale of deed done. They showed us the copy of cheque of rs 10 lacs in favour of my father and instructed us to get the sale of deed register by presenting the copy of cheque in the registrar office. The registration was done in the month of October 2016. After this we asked the bank to disburse us the final amount for which bank asked certain papers from the purchaser Which he kept delaying to give, there was thing big communication gap between the purchaser and the bank and nobody responded to us properly. In month of December 2016 bank somehow managed to gather all the papers from the purchaser and re applied for the loan in the system and re generated a cibil report. Now the report showed additional loan taken by the purchaser in the last two months and it affected the eligibility of the purchaser. Bank has suggested the purchaser to repay the new loan amount. The purchaser is just not agreeing to pay the loan amount and now wants to cancel the housing loan application woh dhfl. In this case what should be done how do we de register the sale deed. What if he doesn't agrees to do so..? They have given us a token amount of 3 lacs via NEFT