specifying a particular person serves a legitimate religious interest. Thus, it is an invalid condition. Whenever they receive less than what was stipulated for them, they may reduce their stipulated duties proportionately. And Allah knows best.
He also said—may Allah have mercy on him:
A Principle Concerning the Conditions People Stipulate in Endowments:
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Some of these involve recompense in this world and the Hereafter, while others do not; some impose hardship on the beneficiary.
We therefore say:
The duties stipulated in endowments devoted to religious purposes fall into three categories. Such endowments include those supporting prayer leaders, callers to prayer, and those engaged in studying the Quran, hadith, jurisprudence, and similar branches of knowledge, or in acts of worship or jihad in Allah’s cause.
The first category comprises acts through which one draws closer to Allah Most High: obligatory and recommended deeds that the Messenger of Allah, may Allah bless him and grant him peace, encouraged and urged people to perform. Such conditions must be fulfilled, and entitlement to the endowment’s benefits generally depends on their fulfillment.
The second category comprises acts that the Prophet, may Allah bless him and grant him peace, prohibited or discouraged. Stipulating such an act is invalid by scholarly agreement, for it is widely reported that the Prophet, may Allah bless him and grant him peace, addressed the people from his pulpit and said: