Dr. Monzer Kahf, a prominent economist and counselor, states:”If a person owns an amount of nisab or more of a Zakatable item and the condition of hawl (one lunar year) is met, this amount is subject to Zakah provided it is not already assigned to either be spent on livelihood or on repayment of a debt.
This means that if the debt is going to be paid from that amount itself (like a debt that is due in a few days after the end of the year, before getting any new earning), it is a debt that is deductible from the amount and if the remainder is nisab or more, this remainder is Zakatable. Otherwise, it is not.
This also means that debts that are not against the amount existing at the due date of Zakah (end of the lunar year) are not deductible from the Zakatable amount at the end of year. The example of these debts is a mortgage on a house that is payable out of future incomes on a monthly basis or otherwise.”