Builder not provided booking receipt and now not returning booking amount.

In January 2019, I had visited a under construction site, where builder site manager gave me option to book/ hold flat by giving 5 lack rupees advance amount in cash and rest other payment to be done after OC received for building and during agreement. If due to any reason booking gets canceled builder will refund whole amount. I request them to take my booking amount by cheque but they insist for cash payment only. So on trust I gave them 5 lack in cash for booking. After giving them cash I ask for payment receipt, they told me I will get receipt at the time of agreement. I insist them to provide me the receipt, so on visiting card they write " 5 lack cash received for flat booking and sign it" and on the cost sheet paper all witnesses sign as a proof of this deal. Now due to covid situation I was not able to complete deal, hence I asked them for refund with mediator(Local Corporator) and 3 witnesses, where builder site manager agree for whole amount refund. Now Builder site manager and his sales assistant accepting that they have taken booking by cash but builder management is not giving money back. meanwhile while they have sale the booked flat without any intimation given to me. I have proof of all conversations with builders sales person and manager where they accepts they have taken money. what should be done in this case. please provide me legal solution.