Builder is asking for extra ₹3 lakh before handing over the flat keys. Is this legal?

I bought a flat in Pune and the builder is now demanding an extra ₹3 lakh, claiming there were additional costs incurred. This demand has come just before the possession date, and he is saying he will not hand over the keys unless I pay. The original agreement did not mention any such additional charges. I feel cheated and don't know what legal steps to take. Can I refuse to pay? What are my legal options?