Builder asking for additional ₹3 lakh before giving apartment possession?

I'm supposed to get possession of my new apartment in Gurgaon next month. However, the builder has suddenly demanded an extra ₹3 lakh, claiming increased material costs, or they won't hand over the flat keys. This was never part of our original agreement and I'm feeling coerced. I've already paid the full amount as per the contract. Is this demand legal? What can I do to ensure I get possession without paying extra?