As per the Hanafi school, 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. Therefore as per the Shaf’i and Hanbali school, if one’s net wealth dips below the Nisab in the year, Zakat will not be due anymore for that year.
And Allah knows best!