Money recovery from builder paid for SRA project

I had purchased an under construction SRA project in Nov 2015 in Dahisar (E) paying 30 % of the amount as per contract before RERA had come into effect. The builder had given a written contract that the money will be refunded with 15% interest in case he is not able to obtain necessary documents like LOI and COC. But till date he has not been able to provide hard copies of any of the necessary documents and neither obtain RERA with all false assurances and new dates. As for the Cancellation deed he bluntly says that 10% will be reduced from principal amount inspite of his failures. Also, the customers who have so far cancelled have not received any complete refund but are given new dates constantly with a change in the scheduled repayment .How is it possible as a customer to get a complete refund of the amount along with the interest lost for these 5 years legally? The Your assistance is much appreciated.