Builder demanding ₹3 lakh more before handing over flat possession, is this legal?

I booked a flat in Pune with a well-known builder, and the possession was supposed to be given this month. The builder is now asking for an additional ₹3 lakh, claiming increased costs. This demand wasn't mentioned in our agreement signed last year. Without this payment, they are refusing to give possession. I'm feeling pressured and unsure if this is legal. What can I do to get possession without paying extra?