Builder demanding extra ₹3 lakhs before registration, threatening to cancel?

I'm in Mumbai and booked a flat worth ₹75 lakhs. The builder is now demanding an extra ₹3 lakhs just before the registration, threatening to cancel the booking if I don't comply. I feel cornered as I've already paid 90% of the amount and cannot afford delays. What legal actions can I take against this unfair demand? Can the builder legally cancel the booking if I refuse to pay the extra amount?