Builder is asking more than agreed amount in Sales Agreement, otherwise he won't register property.

I bought a flat in Kolkata (Dumdum area) with super built-up area of 850 sq. ft. The rate was 200/- per sq. ft., resulting the total amount of 28,05,000/- + GST. The sale agreement was signed in September 2022. I took a house loan from PNB Housing Finance in the same month of amount 25 Lakhs (the rest amount I will pay myself). The disbursement started from October 2022. Builder verbally committed handover by Jan 2023, though sale agreement says 24 months from date of sign. Post that the work is constantly being delayed. Till date 80% of the entire amount is paid (house loan share + my share, which I was to pay myself). Since bank was insisting me to register the property I called the builder today to mutually fix a date. He said, until I make the entire payment and additional 200/- per sq. ft. (for unknown reason), he will not register the property. This additional amount itself totals to around 1.7 Lakh, which is huge. Now I cannot back out from buying it, because then I will have to pay 10% of total amount + GST, as agreed in sales agreement. What should be my steps now? The property is still under construction (brickwork is done) and the project is not RERA registered.