Refund of Booked flat Rera registered project

Respected sir i booked a flat in September 2018 in a newly started rera project G + 21 bldg n was told completion of project 2019 dec as per Rera.I was given allotment letter mentioning flat no ,along with Receipt of payment as i have given 3 cheques of 10 lakh each total cost of flat was 70 lakhs plus Gst ,stampduty registration n other charges n i told them if possible ill do a part payment in December n rest ill pay before possession. I the allotment letter there was a condition for remaining payment in December so i talk to the bldr he said u try so i have not signed the allotment letter only bldr have signed n then it continued ,after 2 mnths he handed me a notice twice each month stating delay payment with gst charges.Then he stopped sending notice when i spoke to him that i have not accepted the payment condition n not commited such condition of payment. Then from june as the work was stopped since booking people also suggested me to withdraw booking amount i started asking refund from builder he said if u have customer u can ask him to book flat n bldr will refund the amount with profit ,n said as soon as we get any booking they will refund ,sir till date nothing have happened n he just gives me asurance ,he is hardly available in office n his staff gives excuses they dont have client,business is down ,builder is out of stn .Now pls help me how to tackle with bldr to get my refund with or without profit.