What is the ruling if a person buys a permissible commodity, and then it becomes clear to him that the purchase contract is void or invalid, and he was unaware of that at the time of purchase, knowing that he cannot return the commodity?

If a sales contract is invalid, it is to be annulled. The buyer returns the merchandise, and the seller returns the price. If the merchandise cannot be returned due to its loss, then the buyer must return its value, and the seller must return the price. The buyer owes the seller the value of the invalidated merchandise, which is to be estimated by experts. The yield of the sold item belongs to the buyer in exchange for the liability, based on the hadith: "The yield is in exchange for the liability."