Builder Flat booked 50% paid but not able to pay the rest

I booked a Flat after getting the allotment through lottery in Feb'14. Subsequently paid about 50% of the mentioned cost over a period of one year by May'15. Due to severe financial constraint as I lost my job, could not pay the balance as demanded from time to time for over a year. Finally the allotment was cancelled by the builder in June'16 last and mentioned forfeiting of 15% of the cost of Flat and returning the balance as per the GTC signed by me. No money has been returned as yet as I have not yet agreed to the forfeiting amount. They have regretted the ability to reduce the forfeited amount in e-mail communications. Kindly advise on how to get the full refund or whether it is possible at all to get the same back.