One of the conditions of zakaah being obligatory on money is that one year has passed since you acquired the nisaab, because of the report narrated by Ibn Majaah (1792) from ‘Aa’ishah (may Allaah be pleased with her) who said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “There is no zakaah on wealth until one year has passed.” Classed as saheeh by al-Albaani in Irwa’ al-Ghaleel (no. 787).
Based on this, if the money dips below the nisaab during the year, either because one sells livestock or some of them die, or there is a dip in the money, then zakaah is no longer obligatory, because if the wealth dips below the nisaab during the year, then one full year has not passed since it was acquired, so no zakaah is due on it.
Then if the wealth reaches the nisaab again, then you have to start counting a new year from when it reached the nisaab.
Al-Nawawi (may Allaah have mercy on him) said in al-Majmoo’ (5/506): Our view and the view of Maalik, Ahmad and the majority, is that with regard to wealth that is subject to zakaah and where one year must pass before zakaah is paid, such as gold, silver and livestock, it is essential that the nisaab be present throughout the year. If it dips below the nisaab at any point during the year, the year is cancelled. If it reaches the nisaab again after that, then a new year must be counted from when it reached the nisaab again. End quote.
Al-Bahooti (may Allaah have mercy on him) said in Kashshaaf al-Qinaa’ (2/179): If the (wealth) dips below the nisaab partway through the year, then it is cancelled, because being above the nisaab throughout the entire year is one of the conditions of zakaah being obligatory. End quote.
Based on this, no zakaah is due on this money that dipped below the nisaab, and a new year should be counted from when you receive the salary that will bring your wealth back up to the nisaab.
For more information on the nisaab for cash, please see the answer to question no. 100570.
And Allaah knows best.