Opinions differ. The Malikis, Hanbalis, and Hanafis hold that debt reduces the zakatable wealth of the debtor by the amount of the outstanding debt. Accordingly, debts are deductible from assets subject to zakat (Fiqh az-Zakat, 90-94).
Shafi’is argue that zakat is wealth under the payer’s control. Therefore, if one has nisab for a zakat-year one still pays zakat on the zakatable wealth in one’s possession even if one’s debts, were they to be deducted or paid, would consume one’s wealth entirely. That is, one pays zakat on one’s eligible wealth, unless one chooses to pay debts before estimating zakat dues.
This position concurs with the statement of ‘Uthman ibn ‘Affan, third Caliph of Islam, who said in a Friday address (khutbah): “This is the month of your zakat. Whoever among you owes debts, pay them back, that you may commence paying the zakat on your assets” (Al-Amwal, 437). In another version reported by Malik, ‘Uthman reportedly said: “Let one in debt repay his debt, then pay zakat on his remaining assets.” (Al-Talkhis, 178) This address came from the pulpit in the presence of many Companions. None objected.