[Ruling on Zakat for One Who Pardons a Despaired-of Debt]

Answer—and Allah is the Granter of success: He is not obliged to pay zakat on what is of that kind. This is the madhhab, as in al-Azhār and its commentary, wherein it says: “And whoever collects a hoped-for debt”—i.e., not despaired of—“or absolves (ibrāʾ) a debt of that sort …” etc..
If it be said: Is the one who absolves a despaired-of debt rewarded?
We say: Yes. Allah the Exalted has made absolving the debtor and remitting the debt a charity. He, Exalted is He, said: “And if someone is in hardship, then [let there be] postponement until [a time of] ease. But if you give [from your right as] charity, then it is better for you.” [al-Baqarah:280]. And in the supplication of Zayn al-ʿĀbidīn—peace be upon him—in the Ṣaḥīfah is what indicates what we have said.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1