It is permissible because it is permitted by custom. If he violates the description, or purchases it for more than that amount, it is not binding upon the principal. If he said: "Purchase a slave for me for one hundred," and he purchased a slave worth one hundred for less than that, it is permissible; for if he had purchased it for one hundred it would have been permissible, so if he purchased it for less, he has added a benefit, so it is permitted. If it is not worth one hundred, it is not permissible, even if it is worth more than what he purchased it for, because he violated his order and did not achieve his objective.

Section: If he is appointed as an agent to purchase a sheep for one dinar, and he purchases two sheep, each of which is worth less than a dinar, the sale does not apply to the principal. If each of them is worth a dinar, or one of them is worth a dinar and the other is worth less than a dinar, it is valid and binding upon the principal. This is the well-known position in the Shafi'i school. Abu Hanifa said: One of the two sheep applies to the principal for half a dinar, and the other belongs to the agent, because he did not consent except to be bound by the liability of a single sheep. Our position is that the Prophet (peace be upon him) gave Urwah ibn al-Ja'd a dinar and said: "Purchase for us a sheep with it." He said: So I went to the market, bought two sheep for a dinar, and came back driving them, or leading them. A man met me on the way and bargained with me, so I sold him one sheep for a dinar. I came to the Prophet (peace be upon him) with the dinar and the sheep, and said: "O Messenger of Allah, here is your dinar and here is your sheep." He said: "How did you do that?" I told him the story. He said: "O Allah, bless him in the deal of his right hand." Also, because he secured for him what was permitted and an addition of the same type that benefits and does not harm, so it applies to him, just as if he had said: "Sell it for a dinar," and he sold it for two dinars, and what he mentioned is invalidated by the sale. If the agent sells one of the two sheep without the principal's order, there are two views: One of them is that the sale is void, because he sold his principal's property without his order, so it is not permitted.