Is it permissible to give the debtor from the Zakat to pay off a small portion of his debt, knowing that the creditor may demand the debt suddenly before the agreed-upon payment due date?

As long as the creditor informed the debtor that he would demand the debt when needed, the debt becomes due upon demand. The general rule for loans is that they are due. If the employed debtor is unable to repay the debt at that time, he is considered among the gharimin (those in debt) and it is permissible to give him zakat. However, if his debt is deferred, and the creditor has not demanded it, and he is able to repay, then he is not eligible for zakat. Jurists differed on giving zakat to the deserving gharim whose debt is deferred, with several opinions: the soundest is not to give, the second is permissibility, and the third is permissibility if the term of the debt expires in that year.