«وَمَا دَرَى مِنْ جَهْلِهِ أَنَّنِي / أَقْضِي غَرِيمِيَ الدَّيْنَ مِنْ جِنْسِهِ»
In his ignorance he did not realize that I settle a claimant’s debt with something of its own kind.
Gharīm can refer to a party to a debt, creditor or debtor according to context; aqḍī al-dayn means to discharge a debt. The poem compares the treatment of a friend to settling an account.
Min jinsihi means “of its own kind.” The central image is that a response corresponds to the treatment received. This is the poet’s figurative argument, not a discussion of the detailed legal rules of debt.