Builder is demanding extra ₹3 lakhs before handing over the flat, what can I do?

I'm in Bangalore and bought a flat from a reputed builder. The possession was promised by July 2026. Now, the builder is refusing to hand over the keys unless I pay an extra ₹3 lakhs, claiming increased construction costs. I've already paid the full amount as per the agreement. This feels like extortion. I'm stressed as I need to move in soon. My family is pressuring me to pay, but I don't want to give in. What are my legal options to tackle this issue in Karnataka?