The obligation of one who repents from deception in sales

The Shaykh : As for those whom he knows, the matter is clear: he should seek their forgiveness and the issue is concluded. As for those whom he does not know, he should give charity on their behalf for the difference between the value of the item when it was sound and its value when it was defective. If he does not know the amount, he should investigate to the best of his ability, and