Builder demanding extra ₹3 lakh before registration in Pune?

I booked a flat in Pune last year and paid the full amount as per agreement. Now, when it's time for registration, the builder is demanding an additional ₹3 lakh citing increased material costs. He is threatening to cancel the booking if I don't pay. I feel trapped and can't afford this unexpected expense. Can he legally enforce this demand? What are my options to fight this?