unlike the binding contract. Once this is established, if he says: "Whoever returns my stray animal or my runaway slave to me, or sews this shirt for me, or builds this wall for me, shall have such-and-such," it is valid and constitutes a non-binding contract, where each of the two parties may withdraw from it before the work is performed. However, if the person offering the reward withdraws before the work is undertaken, there is nothing against him; but if he withdraws after it has been undertaken, he owes the worker the equivalent wage (ujrat al-mithl), because he performed the work for compensation and thus it was not delivered to him. If the worker rescinds the contract before completing the work, he is entitled to nothing, because he has forfeited his own right by failing to perform what the compensation was conditioned upon, becoming like the agent in a Mudarabah contract if he rescinds before the profit appears. The compensation must be known. The difference between it and the work is from two aspects: First, necessity calls for the work being unknown, such as not knowing the location of the stray or the runaway, whereas there is no necessity that calls for the compensation to be unknown. Second, the work does not become binding, so it is not required for it to be known, whereas the compensation becomes binding upon the completion of the work, so it is required for it to be known. It is possible that a reward (ju'alah) is permissible with unknown compensation if the ignorance does not prevent delivery, such as saying: "Whoever returns my runaway slave shall have half of him, or whoever returns my stray animal shall have a third of it." For Ahmad said: If an emir says during an expedition: "Whoever brings ten heads shall have one head," it is valid. They said: If he sets a reward for someone who guides him to a fortress or an easy path, and the reward is from the wealth of the disbelievers, it is permissible for it to be unknown, such as a slave girl designated by the worker. Therefore, the same is inferred here. But if the ignorance prevents delivery, the reward is invalid in all cases. If the work is known, such as saying: "Whoever returns my slave from Basra, or builds this wall for me, or sews this shirt of mine, shall have such-and-such," it is valid; because if it is valid despite ignorance, it is all the more valid with knowledge. If he attaches it to a known duration, saying: "Whoever returns my slave from Iraq to me in a month, shall have a dinar," or "Whoever sews this shirt of mine in a day shall have a dirham," it is valid; because if the duration is permitted while unknown, it is all the more permitted when specified.