Builder demanding extra ₹3 lakh before possession of flat, what can I do?

I booked a flat in Pune with a builder in 2024 and paid all installments as per the agreement. Now that the possession date has arrived, the builder is demanding an extra ₹3 lakh citing increased construction costs. I feel betrayed because this was never mentioned before. I can't afford this additional amount and it seems unfair. The agreement clearly states the total cost, and I've already paid that. What are my legal options to get the possession without paying this extra demand? Can I approach RERA or should I file a complaint in consumer court?