What does an estate trustee actually do day to day?

They are conditions of validity. The wasi must be of age and of sound mind, Muslim, and of established integrity (‘adala — in the Maliki formulation, trustworthiness and the competent handling of a child’s property). An appointment that fails one of these does not take effect at all: a Muslim cannot validly appoint a non-Muslim over his children or his estate, and Ibn Qudama records Malik and al-Shafi’i, with one of the two reports from Ahmad, as holding the same of a person of doubtful integrity. The Shafi’i list adds that the wasi be free, not an enemy of the person in his charge, and not of unknown character; a blind man may serve. The Maliki school adds that a wasi who later loses a qualification — who apostatises, or becomes incapable — is removed.

Sources: al-Fiqh ‘ala al-Madhahib al-Arba‘a, vol. 3, “Mabhath al-wasi al-mukhtar” (the chosen executor), school by school (‘Abd al-Rahman al-Jaziri (d. 1360 AH)) · al-Mughni, Kitab al-Wasaya, fasl 4773: “fi man tasihhu’l-wasiyyatu ilayhi wa man la tasihh” (who may be appointed wasi) (Ibn Qudama al-Maqdisi (d. 620 AH)) — each school’s list of conditions set out separately in al-Jaziri, and Malik, al-Shafi’i and Ahmad separately named in al-Mughni on the person of doubtful integrity.

The appointment stands; the judge fixes it. Where the person named is a minor, a non-Muslim, or of doubtful integrity, the appointment is valid and anything he does before he is removed takes effect — but the judge must replace him with someone qualified, substituting a Muslim for a non-Muslim, and removing a person of doubtful integrity only where he is actually suspected with the money. A wasi who meets every condition may not be removed by the judge at all, because he stands in the place of the deceased who chose him. Ibn Qudama states that a Muslim’s appointment of a non-Muslim is invalid with no disagreement he knows of; al-Jaziri records this Hanafi rule expressly, so the attribution of unanimity is not one we can pass on.

Sources: al-Fiqh ‘ala al-Madhahib al-Arba‘a, vol. 3, “Mabhath al-wasi al-mukhtar” (the chosen executor), school by school (‘Abd al-Rahman al-Jaziri (d. 1360 AH)) · al-Mughni, Kitab al-Wasaya, fasl 4773: “fi man tasihhu’l-wasiyyatu ilayhi wa man la tasihh” (who may be appointed wasi) (Ibn Qudama al-Maqdisi (d. 620 AH)) — the Hanafi conditions, and the valid-but-removable structure, set out in al-Jaziri; Abu Hanifa separately named in al-Mughni on the person of doubtful integrity.