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It is not a condition of a reward arrangement that the work involved be known in advance; it is permissible whether the work is known or unknown.
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The person undertaking the work is not bound to perform it and may withdraw, whether or not he has begun. He is entitled to nothing unless he completes the work. There is disagreement, however, concerning the person offering the reward. According to one view, the arrangement becomes binding on him when the contract is concluded. This is one of Ibn Habīb’s two positions and is the apparent meaning of ʿĪsā’s report from Ibn al-Qāsim concerning reward arrangements and hiring contracts. According to another view, it does not become binding on him until the worker begins the work. This is related by ʿAlī ibn Ziyād from Mālik, and also by Ashhab from him in the section on artisans’ liability in al-ʿUtbiyya. It is also Sahnūn’s position and is the stronger of the two views. Since the worker is not bound by the arrangement, the person offering the reward should likewise not be bound until the worker begins, at which point it becomes binding so that his work is not wasted.
The reasoning behind the first view is that, since what the person offering the reward commits must be known and may not be unknown, the arrangement becomes binding on him. Since the work contributed by the worker in a reward arrangement may be unknown, he may withdraw whenever he wishes and is not bound by it. Consider how a hiring contract, because it involves a known payment for known work, binds both parties, neither of whom may withdraw.
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Accordingly, if the person offering the reward dies before the worker begins—according to Ibn Habīb’s position and the apparent meaning of ʿĪsā’s report from Ibn al-Qāsim—or after the worker begins—according to the reports of ʿAlī ibn Ziyād and Ashhab from Mālik, may Allah Most High have mercy on him—the arrangement is binding on his heirs, and they may not prevent the worker from proceeding. If the worker dies after beginning the work, or, according to one of the two views, before beginning it, his heirs take his place, and the person offering the reward may not prevent them from carrying out the work. Asbagh, however, related a contrary ruling from Ibn al-Qāsim concerning a reward arrangement for