Obtaining bl and ro certificate against loan for purchase of flat

Respected Sir, This is in regard to the purchase of a resale flat in Kestopur area (Municipality :: gopalpur municipality) in west bengal. I want to purchase a flat from a party who is already continuing his loan in a private bank. In this regard I wish to repay his loan amount and proceed for legal agreement after fulfilling the party's monetary demand. Now, the party had submitted the BL and Ro certificate's copy provided to him by the promoter who happened to be the owner of the land on which the apartment was constructed. Now, the party wants to sell his flat but he did not do the BL and Ro mutation although he has paid the municipality tax since his possession on the property after carrying out municipality mutation. Now, for paying rest of the amount I have to take bank loan who is asking for Bl and Ro mutation certificate of the present owner. How can I suggest the owner to get the same. Please suggest any local lawyer (if required) to sort out the matter. Regards, ABHISHEK BANIK