Seller wants to cancel the deal after registration

Dear All, I have bought a flat in the same building where I currently stay. Infact on the same floor, opposite flat. I did the registration, paid the stamp duty and mortgage the property for Loan purpose. Also, did the intimation in the registration office about the same. Now, the bank is ready to disburse the cheque, provided I get the Share certificate and "Declaration cum indemnity bond" from the seller (As the seller do not have original Sale agreement/photocopy, and original Share certificate). He did an FIR and newspaper Advertisement and have the NO Claim certificate from a lawyer. Now my problem is, the seller took the a fair amount of money offline (Hard earned and tax paid from my end) + two lakhs white money as per the agreement, now I need to pay him the rest of the amount within 45 days. However, he is not communicating with me nor picking up my call. He is not allowing me to goto his home and even talk. He came yesterday and said that he wants to cancel the deal. Since the flat was vacant, I started carrying out the renovation work, but he suddenly came and took the keys and threating us to cancel the deal. I've already spent a lot of money offline on that flat, if he cancel I will only get peanut. Someone please advise if I can stop him from cancelling the deal? I need that flat, as I have already spent a lot on registration, transfer fees to the society, lawyer fees, 1% of the loan amount to the bank, legal documents signed for the loan purpose with the bank+mortgage fees + intimation to the registration office, and finally around 2 lakhs on the renovation work so far. Please help me to understand what should I do now :( I'm confused now, as without any issue why is he cancelling the deal.