Is it legal for a builder to ask for extra charges nearing possession date?

I booked an apartment in Mumbai with a builder three years ago. According to the signed agreement, all payments were settled. Now that the possession date is near, the builder is asking for an additional ₹3 lakh for 'infrastructure development'. They are threatening to delay the possession if not paid immediately. I’m really worried and feel this is unfair. What can I do to deal with this situation legally?