Builder refusing to register flat unless I pay ₹3 lakh extra?

Hi, I'm from Pune and I booked a flat with a builder last year. Now he is refusing to register the property unless I pay an additional ₹3 lakh, which was never part of our agreement. I’ve paid 90% of the total cost already. I feel cheated and don’t know what legal steps to take. Can I file a case in the consumer court, or should I talk to the police? Any advice would be appreciated. I'm really stressed about this situation.