Builder demanding extra ₹3 lakh before handing over flat keys, allowed?

I bought a flat in Pune and have paid all the installments as per the agreement. Now, the builder is asking for an additional ₹3 lakh claiming some new development charges which were never mentioned before. He's refusing to hand over the keys unless I pay. This feels like extortion and I don't know what to do. Can he legally demand this extra money? How can I proceed to get my flat without paying this unjust amount?