Interest demand by the builder

We have three documents from the builer: 1. Unregistered agreement for sale of flat - Prepared and signed in June 2015 2. Registered Agreement for construction - Prepared and registered on June 2018 at the time of registration 3. Registered Sale Deed - Prepared and registered on June 2018 at the time of registration. The schedule of payment and the interest was mentioned in the Unregistered agreement for sale of flat but not mentioned in the Registered Agreement for construction. The registered document of sale deed does not state anything about the interest on late payments. The registered agreement of construction states that they have received the entire amount and I am the owner and can regsiter in all the places on my name. Now for the late payments that were made before the registration the builder is claiming inyerest and says he will not hand over the flat if not cleared. Here it states that any late payment we have to pay interest. As it states taht the amount is received and has been regsitered on my account, guess this is for late payments after the registration. I am also sacred to go legally as he may spoilmy flat. Can you please advise the best way to proceed. Also if I pay now, take possession and then go to consumer what are the chances of me getting back the money.