Can builder refuse possession unless I pay extra charges in Mumbai?

I bought a flat in Mumbai and made all payments as per the agreement. However, now the builder is refusing to give possession, demanding an additional ₹3 lakh for 'unforeseen costs'. I am not sure if this is legal or a tactic to extract more money. I am worried this will delay my moving plans. What can I do legally? Is there a way to enforce the possession as per the original agreement?