RERA is almost toothless in all almost all states. Only a lawyer can help you in this. The builder charging interest payment is wrong and is a tool from his end to show his power.
A good lawyer can help to lodge a case in Consumer Forum. It will be a long battle, but worth fighting to get your money/house back.
A lot of builders have used Covid as an excuse to delay the possessions of the house. Even I faced delay in possession but that was a genuine reason. If you have taken a loan from bank, then ask the bank officials if then they can help in any way.
An official complaint at some authority/politician maybe helpful as well. But do consult a lawyer ASAP.
Good Luck. Hope you find resolution.