Developer asking extra money beyond the money agreed in Sale deed

I booked a flat of Super buildup area 976 sq.ft in Aug'15 & already paid 644000/-. As per the sell agreement the total cost of flat is 3220000/-. The flat got completion certificate from local municipality in the year of 2013/2014. As I am taking bank loan, my bank (SBI) assessed the flat in Sep'15 & measure it 976 sq ft also.Now just before I am going for flat registration, they are asking me to pay 43000/- more money as they are demanding that my flat super build up area raised 13 sq ft.As I am buying a already complete flat then how the flat measurement suddenly gone up. Is this demand legal. Also, they calculating super build up area as (build up area + proportionate stair area & lift lobby) then 25% service area over that total. Is it a legal way to calculate super build-up area ( inclusion of stair & lift area to calculate the 25% service area?). they threating to cancel the agreement & will forfeit the amount I already paid (644000/-) if the agreement canceled. plz advice. thanks