Builder demanding extra ₹3 lakh before registration, is this extortion?

I booked a flat in Pune with a reputed builder and we agreed on ₹75 lakh. Now, a month before registration, the builder is asking for an extra ₹3 lakh, claiming increased costs. He's threatening to cancel the booking if I don't pay. I've already paid 90% of the original amount and can't afford more. I'm feeling trapped and extorted. Is this demand legal and what can I do to fight back?