[Expiation (Kaffārah) for Prayer]

Answer—and Allah grants success: If the deceased did not specify the amount for each expiation or for the total, then the executor must pay what the people of that land customarily recognize as the amount for a “prayer expiation”—if such a custom exists. If there is no local custom, then the executor must follow what the scholars of the school have set: one-half of a ṣāʿ for each day and night; thus for twenty expiations the due amount is ten ṣāʿs.
Yes: Prayer expiation is recommended (mandūb) in this school. It is not obligatory upon the deceased to bequeath it. They reasoned that it never became attached to the estate during life—whereas the expiation for fasting does transfer to the estate in case of incapacity. According to Imām Zayd b. ʿAlī—peace be upon him—(as in the marginal notes), it is obligatory.
I say: The expiation for fasting is indicated in the Qur’an and Sunnah, unlike the expiation for prayer; perhaps the scholars of the school analogized prayer to fasting.
One may, however, infer a general basis for expiation of prayer from His saying: “Indeed, good deeds do away with misdeeds.” [Hūd:114].
Thus one may infer the recommendation of expiating by charity, but without specifying a fixed amount.
So, if a deceased person bequeaths an expiation for prayer (or more), the executor should set the amount by reference to the nearest recognized expiation in the law—namely, the fasting expiation—provided there is no local custom for “prayer expiation”; otherwise, the custom is followed as noted above.
Accordingly, if a deceased person bequeaths an expiation (kaffārah)—or more than one—for a sin he committed, it should be taken to mean the least expiation, namely the fasting expiation, which is a half–ṣāʿ, provided there is no contrary local custom, as stated earlier.
If, however, the deceased had a known personal custom regarding the amount of expiation, then it should be carried according to his custom. Thus, if he was accustomed in his lifetime to expiate with twenty ṣāʿs for neglecting prayer or for committing a sin, that custom must be followed in his case, and the executor must pay twenty ṣāʿs for each expiation.
Scholars of the school have mentioned something similar in oaths: they said that the interpretation of an oath is referred to the custom of the one swearing, and so on.
Yes: in this school, “the expiation for prayer” is not a true expiation, so it is permissible to give it to the Banū Hāshim, since it is not the discharge of a debt-like obligation attached to one’s liability; it therefore resembles other acts of pious charity. (End from the marginalia.)
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1