Legal action or solution for assured written default

Dear Sir/Ma'am I purchased commercial retail property in Gurgaon. The project has the approved RERA license number. In the builder buyer agreement they have agreed, without any specific exception or caveat clause, to pay assured return on the entire amount credited until possession. Hence I paid advance money i.e. before the construction linked payment planned milestone achieved. The project got delayed due to many reasons and now the builder seems to be defaulting on the assured return (AR) by delaying monthly payouts of AR. This month (June 2021) they didn't pay assured return and mentioned that they can offer to adjust the remaining assured return amounts at the time of possesion. However (a) I am not happy with this offer due to trust issue (b) I have paid almost 99% of the total cost, so there is nothing to be adjusted now. I don't expect that the builder would refund the extra amount. So I foresee a foul play and default on the BBA agreed clause of AR. I want to know what can be the Legal solution i.e. What are the customer rights and resolutions in this situation. What shall I do to get either the resumption of AR or refund of my advance paid money.