Builder suddenly demanding extra ₹3 lakh before handing over the flat, is this legal?

I booked a flat in Pune and have already paid 90% of the amount as per the agreement. Now, the builder is asking for an additional ₹3 lakh claiming unforeseen costs, threatening to cancel my booking otherwise. This demand came just a month before the scheduled registration. I'm worried and feel trapped because I can't afford this sudden extra charge. What are my legal options to fight this? Can I take any action under RERA?