Property purchaser deceased and advance amount forfeited

Hello there, This is to let you know that, my father had attempted to purchase a floor of a bungalow as a flat occupancy with 3bhk configuration. On 12th August 2020, the property seller had made a non-stamped Memorandum of Understanding (MoU) stating the sellers' and purchaser's identity and the necessary property sale clauses wherein my father signed as the purchaser, and I, being his only son, had signed as the witness in a completely cool and fresh state of mind; the MoU also stated the payment and receipt of Rs.20,00,000/= INR as an advance/earnest payment for the property wherein Rs. 4,00,000/= was paid in cheque and Rs. 16,00,000 was paid in cash (on request of the purchaser to avoid Income Tax deduction; to pay black money). It also states that if the purchaser fails to get the property registered within 6 months of this MoU, then the entire of the advance/earnest payment will not be returned and will be forfeited. But on 24th August 2020, unfortunately my father, the sole purchaser of the property, had passed away for some disease and now, the seller, according to the memorandum, is claiming to forfeit the cash amount of Rs. 16,00,000. Please help....