Builder demanding extra ₹5 lakh before registration, how to resolve this?

Hi, I'm from Pune and bought a flat that's ready for possession. Just before the registration, the builder is demanding an additional ₹5 lakh, claiming increased costs. I already paid ₹50 lakh as per the original agreement. This demand was never mentioned in any written communication and feels like extortion. I can't afford this unexpected expense and need the flat soon. Tried talking to the builder, but he's not budging. What legal action can I take to challenge this? Is there a quick way to resolve this through consumer court?