Is it permissible to inform the insolvent debtor that the zakat debt will be returned to him after it has been collected from him, so that he can repay his debt to me?

If you and the debtor agree that he should return the money to you as zakat, this is a stratagem to revive your money and recover your debt, and this is not permissible. However, if the debtor gives you the amount as repayment of your debt without prior agreement, and then you return it to him because he is one of those eligible for zakat, then that is permissible. Imam Ahmad stated that stratagems in zakat are disliked, and that whoever intends, by giving zakat, to revive his money or recover his debt, it is not permissible. Ibn al-Qayyim mentioned that doing so as a stratagem does not discharge the zakat, but if one gives it as a gift, cutting off his hope of its return to him, and then the debtor repays his debt from the zakat, this is permissible.