Is it permissible to take a portion of the Muslim zakat money and give it to an agent to pay off a debt owed to him by an insolvent person, without the latter's knowledge, and then distribute the remainder to those entitled to it? And is this considered a form of deception?

It is permissible to give zakat to the indebted person (gharim) if he does not possess what would cover his debt in excess of his basic needs, and this falls under the categories of zakat expenditures mentioned in the Quran. It is permissible to pay the zakat directly to the creditor without the debtor taking ownership of it. However, it is preferable to give it to the trustworthy debtor so he can pay it himself. If there is concern that the debtor might misuse the money, then it should be paid directly to the creditor. Charitable organizations are permitted to pay the debts of those in hardship from zakat funds without their permission, although obtaining permission is better. If a person is entrusted with distributing zakat and uses it to pay a gharim's debt for his own benefit, there is a detailed ruling: if the owner of the money has given complete authorization and has no objection to a large sum being given to a single gharim, then there is no harm in that.