Is zakat obligatory on money that is not used, and the mother takes from it then returns it?

Zakat becomes obligatory on money when it reaches the nisab (minimum threshold) and a full lunar year has passed over it. A mother borrowing money does not remove it from its owner's possession; rather, it is a debt owed by her, and zakat is obligatory on it. The owner of the money has the choice to either pay its zakat along with the rest of his money when the year passes, or when he collects it for the past years. Some scholars are of the opinion that zakat on a debt owed by a solvent and willing debtor must be paid when the year passes. The preponderant opinion is that it is paid upon its collection for the past years; so, whenever he collects any amount, he pays its zakat, even if it does not reach the nisab.