Is the zakat on lent money used for trade obligatory, given that the borrower undertook to pay the zakat himself?

Zakat is obligatory upon the lender if the lent money reaches the nisab (minimum threshold) and he must pay zakat on it every year if the debtor is solvent and not procrastinating. However, if the debtor is insolvent or procrastinating, the lender pays zakat on his debt for one year upon receiving it. The debtor's zakat payment on behalf of the lender is not valid unless with the latter's permission. It is not permissible for the lender to stipulate that the debtor pay zakat on his behalf. However, if the debtor voluntarily pays it with the permission of the creditor when the hawl (one lunar year) is due, there is no harm in it. Scholars have differed regarding the zakat of the debtor, and the closest opinion is that he pays zakat on the debt if he has other money not subject to zakat that covers the value of the debt.