Builder increasing claimed carpet area at time of possession

I booked an under-construction apartment in Mumbai in the pre-GST and pre-RERA era. Agreement was signed (on carpet area) in 2015 and Maharashtra VAT, stamp duty was paid. Now builder has received part OC and is asking for full payment for possession. RERA queries 1. Builder is demanding 7% extra payment by claiming that the carpet area has increased since the agreement was signed. This carpet area increase is not reflected in the layout and so this is just a cheating tactic being employed. It seems that he is counting some dry area/chajja as additional billable carpet area and is charging a proportional rate for it while actual area available to me has not increased. Can I lodge a complaint to the RERA authority and hope that they will enforce the agreement that I had signed with the builder? What are my chances? 2. The building is a high rise and has received part OC for the bottom half while construction in the top-half goes on. The builder is giving possession for apartments in the bottom half of the building. Is this legal and is one allowed to stay in a building where the top half is still under construction? Builder is also asking for all payment within 10 days - while clearly there is a dispute on the sum due. How do I tackle that?