Clarification pertaining to non execution of advance receipt-cum-sale-purchase-agreement

I am a widowed senior citizen lady. I had signed an advance receipt-cum-sale-purchase-agreement with a Purchaser, for Rs. 17,00,000 with Rs. 2,00,000 taken as advance. through a property dealer on February 15, 2021 for the sale of my DDA Slum Flat located in Garhi, near of East of Kailash, New Delhi. As per the agreement, the execution was to be carried out within 3 months i.e., by May 15, 2021. Due to the Covid-19 Pandemic in 2021, with mutual consent, the deadline of execution of agreement was extended to July 15, 2021 via exchange of Whatsapp messages, but not on the actual legal papers. The purchaser did not respond in any way during that period, inspite of our consulting him again and again. Even after expiry of the extended period of this agreement, we had time and again been requesting him to clear the rest of the due amounts and take possession of the said property. Now, it had been almost 1 year and 3 months since the date of signing the agreement, but he has not executed the agreement still. Although as per the language of agreement, we should forfeit his advance amount, we still requested him to either execute the agreement completely or take his advance amount back. But he refuses to complete the deal or take back the Advance / Bayana amount. I believe that the Purchaser is also a property dealer, because suddenly now he has come with a new Party, that wishes to purchase the flat for even more than Rs. 17,00,000 and wants us to take all the money from this new Party, and pay him whatever money we receive over and above Rs. 17,00,000 from this deal. We requested him to first pay us the balance amount of Rs. 15,00,000 immediately to execute his own agreement with us and then he may sell the flat to whoever he wishes to, because we had signed the agreement with the Purchaser and not the new Party he has brought in to purchase the flat. I don't think we should sign a fresh agreement, when even the previous agreement has not been executed by the first Purchaser. If the advance receipt-cum-sale-purchase-agreement has expired due to non-payment of dues within the stipulated time frame mentioned in the agreement i.e., May 15, 2021, then can we forfeit his advance amount / bayana, and sell the said property to somebody else on our behalf? Can he take any legal action against the fresh deal with our own customer?