How is Zakat on money (2.5%) calculated correctly in case of its increase or decrease? Is Zakat calculated on the original amount or the amount after the increase? Is Zakat due on money every year if the amount remains constant?

Zakat (definition) is obligatory on money (gold, silver, or circulating currency) if a year has passed on it and it has reached the nisab (definition) (minimum threshold). So, if you had 100,000 and a year passed on it and you paid its zakat, then it became 150,000 the following year, zakat is obligatory on the entire amount. Zakat is also obligatory if the amount did not increase but remained at the nisab. The current nisab for banknotes is the equivalent of 85 grams of gold or 595 grams of silver, estimated by what is most beneficial for the poor.