As for the jurists who differentiate between the two cases—permitting compensation for a vowed fast but not for one that was originally obligatory—they hold that the hadith of this chapter, along with the following report from Ibn ʿAbbās, must be read in light of the second narration from Ibn ʿAbbās mentioned herein.

Ibn al-Qayyim championed this position in his works Iʿlām al-Muwaqqiʿīn and Tahdhīb al-Sunan, declaring it the fairest of the opinions and affirming that the words of the Companions support it. He explained that Ibn ʿAbbās’s statement, “No one should fast on behalf of another; instead he should feed (the needy) for him,” refers to the fast that was obligatory in its own right. A vowed fast, on the other hand, may indeed be observed for the deceased. Likewise, the report that ʿĀʾishah ruled a charitable feeding for a woman who died owing a fast concerns an original obligation, not a vow.

In this way the various narrations harmonize, and this agrees with both textual evidence and analogical reasoning. A vow is not imposed by the Sacred Law itself; rather, the person voluntarily makes it binding upon himself, so it becomes like a debt he has taken on. For that reason the Prophet ﷺ likened it to a debt in the hadith of Ibn ʿAbbās.

He (Ibn al-Qayyim) further said: The secret of the distinction lies in the fact that a vow is a commitment the legally responsible person places upon his own liability; the Law did not oblige him to it from the outset. Consequently, it carries a lighter ruling than a right the Law has established for Allah over a person, whether he wishes it or not. A liability (dhimmah) can encompass both what one is able to do and what lies beyond one’s ability, whereas the duties laid down by the Law are proportionate to the body’s capacity. End of his summarized statement.

Benefit
According to the majority of scholars, a guardian’s making up the fast on behalf of the deceased is recommended, not obligatory; the Ẓāhirīs, however, consider it compulsory. The Hanbalis rule that if the deceased leaves an estate, the fast must be made up; otherwise it is merely recommended. They also state that if someone other than an heir undertakes the fast, it still fulfills the requirement.

Hadith 8
ʿAbdullāh ibn ʿAbbās (may Allah be pleased with him) reported:
“A man came to the Prophet ﷺ and said, ‘Messenger of Allah, my mother has died while she still owed a month of fasting; should I make it up on her behalf?’ He ﷺ replied, ‘If your mother had a debt, would you settle it for her?’ The man said, ‘Yes.’ He ﷺ then said, ‘A debt owed to Allah has even more right to be settled.’”