Dr.Monzer Kahf, a prominent economist and counselor states: “The principle is: on the day when Zakah is due (354 days after you have Nisab that is about Euro 1000) one should deduct any debt on them that they expect to pay out of the Zakatable assets they have on that day. However, If the payer of zakah expects to pay the debt from other sources (like future monthly income) you should not, then deduct the debt.”