Is it permissible for someone who has zakat and has not handed it directly to the poor and deserving person to pay off their debt with their knowledge?

It is permissible for a debtor who is unable to repay his debt to take zakat to settle his debt, provided that his debt was incurred for a permissible matter, because he falls under the category of gharimin (those in debt). If the debtor authorizes you to pay off his debt from zakat, then there is no objection. However, paying off a debtor's debt without his permission is a matter of scholarly disagreement regarding its sufficiency for zakat.