Praise be to Allah.
If the amount of money reaches the minimum threshold (nisaab) – which is the equivalent of 85 grams of gold or 595 grams of silver – and a full hijri year has passed since acquiring it, then zakaah must be paid on it, even if something is taken from it each month, because of the general meaning of the evidence that zakaah must be paid on wealth that is subject to zakaah, when one year has passed.
If we assume that this money reached the nisaab at the beginning of the year, then it fell below the nisaab during the year, no zakaah is due on it, and the reckoning of a new year should begin when it reaches the nisaab again.
For example: if the amount of money reaches the nisaab in Ramadaan, then it falls below the nisaab in Dhu’l-Hijjah, then you acquire more money which brings it back up to the nisaab in Rabee’, then you should start reckoning a new year from Rabee’.
Al-Nawawi (may Allaah have mercy on him) said: Our view and that of Maalik, Ahmad and the majority is that is in order for zakaah to be obligatory on wealth that is subject to zakaah and for which one year must have passed, such as gold, silver and livestock, the nisaab should have been present throughout the year. If it falls below the nisaab for a moment during the year, the year is cancelled out, and if it reaches the nisaab again after that, a new year should be calculated from the moment when the nisaab was reached again. End quote.
And Allaah knows best.