Buyer Sending False Legal Notice

Legal notice issued to my father by the buyer of my property claiming that around 7.5 lakhs that he sent to my father and mother are a loan and that has to be refunded . Details of property bought and sold: Purchase date : 3 aug 2010 Purchase value : 45.5lakhs Sale date : March 2 2018 Sale price : 59 Lakhs Registered by buyer for 49.5 lakhs Since we want to get the money in white, the money was received in multiple chunks to my account , my fathers account and mother's account from the buyers account, his wife and son totaling 57 lakhs and 2 lakhs as cash. Since buyer registered for 49.5 lakhs he claims that the 7.5 lakhs is a loan he gave in addition to my father for his urgent expenses and for sending me abroad ( btw, I was only visiting India and had a return flight ticket ). It's a clear case of fraud and we want some guidance on how to tackle this buyer and legal notice issued. ( all money was part of the real estate transaction and there is no loan involved ) Thanks !