Is it permissible to give zakat to a poor debtor, by deducting his debt from the zakat amount after informing him of that, if the giver of zakat is one of his creditors?

It is permissible to give zakat to a debtor if their debt is for a permissible matter, because they are considered a "gharim" (one in debt), and this is one of the categories for zakat mentioned in Surah At-Tawbah (9:60). This includes debts owed to the zakat payer or to others. However, it is not permissible to directly deduct zakat from the debt; rather, the zakat must first be transferred into the possession of the debtor, and then they may use it to repay their debt to the zakat payer.