How is zakat calculated on money that reaches the nisab, but fluctuates with increase and decrease during the hawl (one lunar year), while the money is being saved to buy an apartment?

Zakat (definition) is obligatory on money saved to buy an apartment if it reaches the nisab (definition) (minimum threshold) and a hawl (one lunar year) passes over it without decreasing below the nisab. The hawl is calculated from the date the money reached the nisab. As for salary zakat, it can be added to the saved money and paid when the hawl for the saved money is due, or a separate hawl can be calculated for each salary from the date it is received. The easiest and best method is to pay zakat on all savings, from salary and other sources, when the hawl is due on the first nisab acquired.