What should I do with the money I was paid for repairing my car, when it returned to its original state without being repaired, and I cannot return the amount because I do not know the owner of the money?

If the harm for which a settlement was made disappears on its own without treatment, you have no right to compensation, and you are obliged to return the money to its owner unless he forgives you. If you cannot reach him, then give it in charity on his behalf. If a person claims a right and settles for something, and then it becomes clear that the right is not binding on him, the settlement becomes void, and he has the right to reclaim the settlement amount. Similarly, if the defect for which a settlement was made disappears on its own, the settlement becomes void, and the settlement amount must be returned. And if the claimant was in the wrong, the settlement amount is not lawful for him religiously, unless the defendant hands it over to him willingly, in which case it becomes a gift.