What should I do, as I have repented, but do not know the whereabouts of the person from whom I took money?

If you committed a sin by taking money, you must repent and seek forgiveness, and diligently search for the owner of the money to return it to him and seek his pardon. If you do not find him, then search for his heirs. If you do not find any heirs, then donate the money with the intention that its reward be for its owner. If the owner or his heirs appear later, inform them of the matter. If they are satisfied, then so be it. Otherwise, pay them their right, and you will have the reward of the charity.