Is it permissible for the one in charge of zakat to give most or all of the zakat money to a person who owes him money, so that his debt may be paid with this money?

The debtor is considered one of the categories eligible for Zakat if his debt is for a permissible matter. However, if his debt is for something unlawful, Zakat may not be given to him unless he has made sincere repentance.