Since the building has not been constructed yet and therefore all clearances including BCC, CC, OC have not yet come, therefore you can approach the RERA office.
If the building has not received the OC, then you can do the following:
1. You should collect the papers evidences relevant to your case;
2. You should collect evidences of advance payment made by you, including the agreement copy, make sure it is signed by both the parties, the banking transaction showing that you paid the advance amount for the booking;
3. You should write a personal letter to the the builder asking him to not back out from the transaction and hand over the possession, otherwise you could take legal steps;
4. If they still don’t respond, then issue a legal notice through a lawyer to them stating that you have been
wrongfully been cheated by the builder;
5. Since the OC has not yet come, you can also file a complaint before the RERA authority even if the project is no RERA registered;
6. Then last stage you can file a case in court whether a suit or consumer forum or an FIR in police station for all the unlawful and illegal activities of the builder;
7. There is a good chance the builder is cheating you by delaying the process.