Assignment Agreement

Hello, We are buying a flat from a relative who has invested in the flat but did not register it in his name. He has paid the full amount to the builder and made an assignment agreement between him and the builder indicating the concerned flat is assigned to him and the amount he has paid. Our relative wants the loan amount from the bank to be given to him and not the builder as he had paid the builder already in full. When we provided the assignment agreement between the builder and my relative to the bank for processing the loan, the bank inturn has asked me to provide the agreement between my relative and me for them to process the loan. Could you please advise the type of document that has to be made now between me and my relative in order for the bank to process the loan and credit it directly to my relative's bank account. Will it be the assignment agreement between my relative and myself or will it be some other type of document to be made. Could you also advise the process of registering this flat in my name. Thanks Boopathy