Can builder demand extra ₹3 lakh for registration after full payment?

I recently purchased a flat in Pune and paid the full amount as per the agreement. But now, just before the registration, the builder is demanding an additional ₹3 lakh, saying it's for 'extra charges' that were not initially disclosed. I feel cheated and don't understand if this demand is legal. The deadline for registration is next week, and I’m worried he might cancel the allotment if I refuse to pay. What can I do to protect myself in this situation? Is there any legal action I can take?