Builder demanding extra ₹5 lakh before giving possession, is this legal?

I am from Ahmedabad and booked a flat with a builder two years ago. The agreement stated that the total cost was ₹50 lakhs. Now, just a month before possession, the builder is demanding an additional ₹5 lakh, claiming increased costs. I have already paid 90% of the agreed amount. He threatens to cancel and forfeit my deposit if I don't comply. What legal action can I take to stop this harassment?