What kind of document I request from owner of apartment

Respected Sir/Ma'am, I am going to purchase a 2BHK apartment, which is already owned by someone who is living there (other than a builder). So it is a case of second buy. Current owner has already due of around 5 Lakh with HDFC home loan. Now I have gone to HDFC bank to request them for my Home lone for purchasing the same property. They asked me about name and address of the property. I have given information about the same. So in reply they told me that they can not give us loan for that property because the builder (who is not current owner) have not paid the project loan. This same reply was given to me from Axis bank and SBI bank for the same property. They even told me that they will not give me loan for any other building scheme which of same builder. Now the person who is current owner claims that there is no such case and they have all the documents. Now as I really love the place and wanted to buy, what documents do I needed to check from current owner and show it to bank that they can approve. Or where do I go to further check if this is safe to buy.? Thanks in advance.