Can the builder alter the terms and conditions while giving possession

Dear Sir, i bought an under construction flat last year, for which builder has obtained OC now. They are sending Fit out Possession letter to all buyers and asking for complete balance amount. I have following queries: 1. Is the fit out possession letter same as offer of possession. Is it safe to pay all amount now ? 2. As per BBA builder was supposed to penalty for delay, but he says OC was already applied well in time and delay was from Authority side to grant OC, so no penalty is payable, is that correct way ? 3. He promised some fitments (counter top wash basin, etc) also mentioned in booking form. Now he says he will not give, neither will he reduce cost. What do we do ? 4. Lastly he says only 4% ITC will be passed and we need to pay 8% GST, is there some percentage decided by govt on ITC or builder can choose whatever rate he wants ?