1. Have you taken possession of the said property?
2. Pay the amount to the builder under protest, take possession of it and get your property registered,
3. After that file a complaint case before the local District Consumer Dispute Redressal Forum alleging deficiency in service and unfair business practice claiming refund of the extra amount extorted from you with interest, damage and cost,
4. It is a good case to win,
5. If you refuse to pay the amount now, the builder will refuse to handover and register the flat in your name causing huge burden in you.