The counting of zakah year shall start the day in which you receive wealth equal to nisab. If you own wealth equal to nisab on the day when you completed one lunar year, you will have to pay zakah. If the nisab fluctuates during the year, it may not have any effect.
Therefore, the day you become owner of nisab, if you have wealth equal to nisab exactly the same day after completing one lunar year, you will have to pay zakah. The decrease in nisab during the year due to expenditure may not have an effect.
Allah knows Best!
Darul Uloom Deoband