Builder tried to sell the apartment to me which was already sold to buyer A

I am planning to buy an apartment in Bangalore. I will be going for home loan. I finalized the apartment after visiting it for many times in the past 7 (September 2023-March 2024) months and getting the legal documents verified. I have done the due diligence (as much as was possible from my side). I have already made the payment of 15% of the property value to builder and executed the sale agreement in end of April 2024. Then, the sale agreement was submitted to bank along with other documents, during the home loan processing (June 2024) it was found by bank that there is already a buyer A. The buyer A has defaulted on his loan. Now bank has refused to process the loan for me. The builder had not informed me about the buyer A until the bank found it out. Now, I don't want to go ahead with purchase of the property. I have already informed this to builder. Builder is refusing to pay back complete 15% of the amount that I have paid to him during the sale agreement, even after repeated reminders and calls. Now, - What documents should I request the bank to provide to me ? - In the sale agreement there is no mention in the terms and conditions of what happens if the fault is caused by the builder (as in this case). However there is a clause in sale agreement stating that if the purchaser cancels the sale only 50% of the amount will be refunded to purchaser. - In the sale agreement that I have signed it is also mentioned that there is no encumbrance or loan on the property (This is not truth as the bank found out) - What action I can take to get the complete 15% of the money back ? Could you suggest on further actions that I could proceed with ? I want the complete 15% amount back.