[Zakāh on a Recoverable Debt]

Answer: The man should calculate the zakāh due on that debt for the past years. Once he knows the amount, he should release his brother from what remains after deducting the zakāh amount. As for the zakāh itself, it remains a debt—and no further zakāh is due on it thereafter. If the man forgives his brother and releases him from all of it, then the one who pardoned must pay the zakāh; it transfers to his liability.
If these two brothers wish to avoid the zakāh burden, it is permissible for them—given their poverty—to pass a small amount back and forth between some of their family with the intention of zakāh, until their liability is cleared. The scholars of the school have permitted such a device for two poor persons to discharge obligations (not for one who is rich).
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1