Is there a difference of opinion with regards to the passing of a year?
According to the Hanafi school, it is not necessary to own Zakatable assets above the Nisab for the entire lunar year. Zakat will be payable as long as one owns wealth equalling the Nisab on the Zakat anniversary . Fluctuations of wealth during the year will be overlooked, unless any fluctuations reduce a person to zero in terms of net wealth or below zero and in debt.
However, the Shafi’i and Hanbali school condition that Zakat is only payable if one owns wealth equal to the Nisab or more for the entire year.
And Allah knows best!