Builder demanding ₹3 lakhs more before handing over flat keys, is this legal?

I booked a flat in Bengaluru and paid the entire amount as per the agreement. Now, the builder is demanding an additional ₹3 lakhs, claiming unforeseen costs, before he hands over the keys. I am supposed to move in next week, and this extra demand is stressing me out. Can he legally do this? What actions can I take to resolve this issue quickly?