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

I booked a flat in a new project in Pune and have been paying as per the agreed payment schedule. Now, just before the handover, the builder is demanding an extra ₹5 lakh citing 'increased construction costs'. This was never mentioned in our agreement. I refused to pay, and now he is threatening to cancel the booking and keep the money already paid. I feel cheated and trapped. What legal action can I take to ensure I get possession without paying this illegal demand?