Builder demanding ₹3 lakh extra for unforeseen costs, what can I do?

I'm in Pune and the builder of a flat I booked is now demanding ₹3 lakh extra, citing increased construction costs. I have already paid 90% of the agreed amount and the project was supposed to be completed by March 2026. Now they are threatening to cancel the booking if I don't pay up. I feel trapped and don't know how to proceed. Is this legal? What are my options to fight this extortion?