Is zakat obligatory on money that a person lends to his brothers and acquaintances, even if he is not certain of its return to him?

Whoever has a debt owed by a solvent person, reaching the nisab (minimum threshold for zakat) or completing it, then zakat is due on it. He should pay its zakat upon receiving it for the past years. If he pays its zakat before receiving it, that is good. However, if the debt is owed by an insolvent person, he should pay its zakat upon receiving it for one year only, even if more years have passed.