Ruling on making
    use of and investing money with which one has been entrusted
Question

                Someone entrusted some money to me and I
      made use of this money by investing it. When the owner of the money came, I gave him all
      of his money, but I did not tell him how I had made use of it. Was what I did permissible
      or not?
Praise be to Allah.If anyone entrusts something to you, you do not have the
    right to use it without his permission… you have to keep it and take care of it in
    the manner that is usual. If you make use of it without his permission, you have to seek
    his forgiveness. If he forgives you, then all well and good; otherwise you have to give
    him the profit made with his money, or try to reach some agreement with him to take half
    each, or some other agreement. It is permissible to make agreements and reconcile among
    Muslims, except for agreements that make something that is halaal haraam or vice versa.