regestration of receipt of consideration money

We purchased a flat at pune and executed a conveyance deed which provided that xyz amount will be paid by end of next month. My father took housing loan and paid complete amt. Now we are selling this flat but our customers bank is asking to register a document with registrar between my father and person from whom we purchased the flat for receipt of balance cosideration which we paid by loan What exactly is to be done in this case. Can any one provide draft of the document required to be executed? Is it required to be registered with registrar or printing on stamp paper is sufficient.