Builder asks to return agreement document on cancellation without specifying money refund date

Hi, We have booked a flat in Mar 2019 and signed an agreement after paying 20% in May 2019. Builder hid that they received Rera approval and asked to pay 20% for agreement although as per Rera we were supposed to pay 10%. We realized a couple of months later and then they said they would adjust in the next installment when the basement is complete. The basement was supposed to be completed in October 2019. However, it isn't complete yet. So we requested for cancellation in June 2020. 1) We were asked to sign a cancellation agreement and return the actual agreement. 2) This cancellation agreement only mentions the amount that would be returned but no details regarding when it would be refunded 3) 10 % cancellation charges (plus 18% service charges) are applied over the 20% amount we paid. We were charged extra and then being penalized for that 4) As per the agreement, builder would return the cancellation amount in 90 days or when they find a new buyer whichever is later. Is this a valid clause? Builder hasnt made any progress from february 2019 and given current situation finding a new buyer for a project scheduled to be delivered in 2025 doesnt seem easy. We asked what if they dont find a buyer in another year for which they dont have answer. What can we do to get our money back ASAP?