Is it permissible for the questioner's father—in his capacity as director of a charitable organization—to deputize another person to sell a car donated by a benefactor to the organization, with the donor's knowledge of its sale and the allocation of its price as an endowment for him, but without his knowledge of the deputization?

Your father's action of appointing another person to sell the car is permissible. This is because the donor had granted him an absolute agency, and he is the director of the association. It is customary for a director not to handle every aspect of the association's affairs personally, especially since selling cars requires someone skilled in bargaining. This is not among the responsibilities of the association's director, nor of its original employees. Consequently, it has become customary to delegate others in such matters, and "custom is authoritative." Therefore, if an action is not typically performed by the agent, the authorization extends to what is customary regarding delegation in such a matter.