Builder refused possession until I pay extra ₹3 lakh for special charges?

I booked a flat in Pune in 2024 and was promised possession by June 2026. But now the builder says I must pay an extra ₹3 lakh for 'special development charges' before I can take possession. This wasn't mentioned in the agreement. I've already paid the full amount as per the contract. I'm feeling trapped and don't know what to do. Can I take legal action against the builder for this unfair demand?