Giving charity on behalf of others
Question

                A number of years ago I took
      one hundred Saudi riyaals from each of a number of colleagues. Later on I travelled to
      another region and forgot these colleagues, and they forgot me. Now I do not know where
      they are. What should I do about the money I owe them?  Please tell me, may Allaah reward
      you.
Praise be to Allah.If the case is as you describe in your
    question, and you have forgotten the person who lent you one hundred Riyaals, then what is
    prescribed in Islam is that you should give an equivalent amount in charity on their
    behalf. When you remember any of them, then pay him what you owe him, unless he accepts
    the fact that you gave charity on his behalf. This will clear you of debt and bring reward
    to you and to them.