Is Zakat obligatory on money that has reached the nisab, but a hawl has not passed over it due to a sum being borrowed from it one month before the completion of the hawl, and then the sum was returned two months later?

Zakat becomes obligatory on wealth when it reaches the nisab (minimum threshold) and a full hawl (lunar year) passes over it. The majority of scholars hold that debt prevents the obligation of zakat on internal wealth (gold, silver, currency, and trade goods) if the debtor does not possess other surplus wealth beyond their basic needs that could be used to offset the debt.

If the borrowed money causes the amount to fall below the nisab, then there is no zakat on it, and the hawl is interrupted. The hawl recommences when the wealth reaches the nisab again. If you willingly choose to pay zakat for this hawl, it will be greater in reward and purify your wealth more. However, if you do not do so, there is no blame upon you.