All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.

Waiving a debt owed by a debtor, with the intention of counting it toward zakāh, does not fulfill the zakāh obligation. Rather, it constitutes an ordinary act of charity, for which Allah rewards the giver generously — since one of the conditions for the validity of zakāh is that the person paying it must form the intention of zakāh at the actual moment the wealth is paid out. In this case, the debt has already been established as a standing obligation upon the borrower, so it is not valid to simply redirect the intention toward zakāh after the fact.

It is preferable, instead, for the creditor to first collect the debt from the debtor, and then return to him the amount of money he is required to pay as zakāh — or a portion of it — if the debtor happens to fall under one of the eight categories that Allah, the Exalted, has commanded be given from zakāh.

It is stated in ‘Umdat as-Sālik, from the books of the Shāfi‘ī school: "If someone gives [money] to a poor person, stipulating that the recipient return it to him in payment of a debt owed to him, or says, 'I am making what you owe me into zakāh, so take it [as such]' — this is not valid. But if he gives it to him with the [unstated] intention that the recipient will use it to repay him, or says, 'Repay what you owe me, so that I may [then separately] give it to you as zakāh,' or the debtor himself says, 'Give it to me, so that I may [then] repay you with it' — this is valid, and there is no binding obligation for the recipient to actually fulfill this [informal understanding]." And Allah, the Exalted, knows best.