Builder asking additional area payment not included in registration

A builder name Marathe Group under whom a project named Marathe Empress is under construction is cheating the buyers of his flat. this is located at titwala near Ganesh mandir. My Father has been asked to deposit additional amount for extra area that was earlier not part of the built up area. This is asked after registration of the bought house with the old carpet area. When I asked him about the registration for the same, he is not able to answer. Also, because of this he is not giving us the VAT and service tax receipt for which my dad had done payment long back. May be this was the reason for holding back the receipt for taxes. He is now planning to adjust the tax amount given into the extra area and later on demand for additional tax amount He has also changed all his contact numbers.