Is it permissible for a relative to designate someone to possess his wealth after his death, and what is the concept of a gift (hiba), especially if the deceased has a brother and has authorized this person to directly manage the money as a gift?

If your relative gifts you money contingent on their death, it is considered a will (wasiyya). A person is permitted to bequeath up to one-third of their wealth to someone who is not an heir. However, if the bequest exceeds one-third, or if it is made to an heir, it is contingent on the approval of the other heirs. But if your relative gifted you the money while they were in good health, and you took possession of it, then it is yours, whether the heirs approve or not. For a gift (hiba) to be valid, the giver must be of sound mind and mature, and the recipient must take possession of the gift before the giver's death or bankruptcy.