Validity of a purchase when a principal authorizes an agent to buy one sheep for a specified price, but the agent buys two sheep where each is valued less than the price?

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If an agent is commissioned to buy one sheep for a dinar and purchases two sheep, each valued less than a dinar, the purchase does not bind the principal. The transaction is valid and binding upon the principal only if each sheep equals the specified price, or if one equals the price and the other is less. This is the established position within the Shafi'i madhhab. The evidence supporting the permissibility of exceeding the specified quantity when the value is met or exceeded comes from the narration of 'Urwah ibn al-Jald: the Prophet, peace be upon him, gave him a dinar to buy one sheep. 'Urwah bought two sheep for that dinar, then sold one of them for the same dinar, returning the dinar and the remaining sheep to the Prophet, peace be upon him. The Prophet then prayed for a blessing upon his transaction. Furthermore, the agent has procured what was authorized plus an addition of the same kind that is beneficial and causes no harm, thus the transaction is valid, analogous to authorizing a sale for one dinar and the agent selling it for two dinars.