Refund of money for relaunch property

Hello sir/madam, I have invested in one of the prelaunch property which is 40 storeay builing from one of the reputed builder. The prelaunch was on terms that the work will strat in 2 years when they will get IOD and development agreement. If they fail to do so they will rfund our money with 15% interest back to us. From last two years we are waitning for IOD, but recently due to RERA the builder send us the letter saying if you want to refund or transfer to other project, do so as there would be infinite delay in the project. Now as per Allotment letter we should get 15 % interest on our money, since he is asking us to refund. But now he is saying that he will pay us after 2 years. That means we waited for last 2 years for the flat and now he is asking us to wait for more 2 years to get the refund. It is injustice to us. It is mentioned in the allotment letter that any termination/cancellation the refund will be in 6 month. So how can we proceed further and can we get our money in 6 months. Your help in this regard would be very helpful as around 400 people had invested in this project. THe builder collected 30% of flat cost from all of us whihc is around 20 lakhs per person. Regards, Helpless buyer