[Exegesis of the ḥadīth: “This charity is but the refuse of people’s wealth”]

Answer—and Allah is the Granter of success: The Prophet—May Allah bless him and his family and grant them peace—called charity (zakāt) “the refuse of people’s wealth” in view of the fact that Allah the Exalted described zakāt as a purifier in His saying: “Take from their wealth a charity by which you purify them …” [al-Tawbah:103]. Since zakāt purifies its payers from impurities, the Prophet—May Allah bless him and his family and grant them peace—was justified in describing charity as the “refuse” of people’s wealth.
Calling zakāt a “purifier” is figurative usage, and the Prophet’s describing it as “the refuse of people’s wealth” is an extension of that metaphor; for the speech of the Messenger—May Allah bless him and his family and grant them peace—here follows and explains the speech of Allah, such that the two statements are as though connected, and the rhetorical relation in this figurative usage is simile.
Now, because zakāt is a due upon the wealthy that they are obligated to pay, the time comes when divesting themselves of it becomes immediate, and harm and detriment befall them if they do not pay it—therefore it is apt to liken it to filth, which one’s innate disposition requires casting off and keeping away from, the remaining of which and failure to cast off bring harm and detriment.
This metaphor has benefits:
1 - Increasing the exhortation of the wealthy to rid themselves of zakāt and dissuading them from withholding it.
2 - Dissuading people from consuming zakāt and turning back the gaze of those who look toward zakāt though they are not its rightful recipients.
3 - Indicating that it is more fitting for the poor to show restraint in taking it.
As for his saying, “It is the refuse of people’s wealth,” this is not, in truth, the effective cause (‘illah) for its being unlawful to him and to his family, May Allah bless him and his family and grant them peace. If that were the cause of it being unlawful to him and his family, May Allah bless him and his family and grant them peace, it would be unlawful to all people, since consuming what is foul is forbidden to everyone.
The true cause for its being unlawful to the Prophet—May Allah bless him and his family and grant them peace—and to the People of his House is that taking it is a locus for suspicion and for the entry of doubt concerning his Prophethood. Hence the Qur’an reports from many prophets words to the effect of: “Say, ‘I do not ask you for it any payment.’” [al-Anʿām:90].
If it be said: How do you depart from the stated cause in the text—his saying—May Allah bless him and his family and grant them peace— “It is but the refuse of people’s wealth”—to a cause for which there is no explicit proof?
We say: The cause mentioned in the text is expressed by way of metaphor; zakāt is not literally the refuse of people’s wealth, but rather it was likened to refuse. The unifying rationale appears to be this: had the Prophet—May Allah bless him and his family and grant them peace—consumed zakāt, his reputation would have been sullied by suspicion, a defilement akin, in its offensive nature, to tangible filth. Thus what we have mentioned as the cause is, in essence, the very cause indicated in the text.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1