What are the permissible arrangements for compensating an agent collecting Zakat (Sadaqa)?

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The Imam has the discretion to either contract the collector (Amil) with a valid wage (Ijarah Sahihah) for a known period or for a known task, or to assign a known compensation (Ja'al) contingent upon the completion of the work. If the work is completed, the stipulated amount is earned. Alternatively, the Imam may send the collector without specifying compensation beforehand and then pay him afterwards. This permissibility is supported by the action of Umar (may Allah be pleased with him), who stated that the Prophet (peace be upon him) sent him to collect Zakat, and upon his return, the Prophet compensated him. Umar then requested that the payment be given to someone more in need than himself. This arrangement is permissible.