Seller Not Registering The Flat In Buyer's Name

Hello, anyone can please advice? I had selected a resale flat in Bhayandar (West) in Dec. 2024 via a local broker & paid 77 lakhs (flat value 78 lakhs & 1 lakh to be paid on registration completion) via bank RTGS to the seller as of 23-12-2024 however the seller has been delaying the registration giving multiple excuses. We had agreed for physical registration on 23-12-2024. I have also paid the stamp duty and registration online in Dec. 2024. The flat is in joint name of father and son. The seller is also not willing to pay back the 77 lakhs paid in order to cancel the deal. The broker has informed me recently that the seller has mortgaged the flat in question for 2.25 crores. What should be the next steps in this case? How long to wait if there is a time frame?