Is it permissible for a relative to bequeath his money to me exclusively after his death, and what is the concept of a gift (hiba), knowing that he has a brother and has appointed me to manage the money as a gift from him?

If the gift is conditional upon death, it is a will, permissible within the limits of one-third for a non-heir, and enforceable for more than one-third or for an heir if the other heirs permit it. However, if the gift was made while the donor was in good health and the donee took possession of it, then it is valid, and the heirs have no right to object to it. For the gift to be valid, it must be from a mature, sane person, and the donee must take possession of it before the donor's death or bankruptcy.