Since Ramadan, I have an amount of money and reached the Nisab {the minimum amount liable for Zakah}.

Three months later, I got another amount.

Is zakah due on the whole amount or due on the part that just reached the Nisab which only one lunar year passed?

Answer:

      The preponderant opinion is the opinion of the Shafi’is and Hanbali’is. Thus the questioner should pay zakah on the money which one lunar year has passed, and a separate lunar year should be calculated for the new earned money.

The prophet (PBUH) said, "Zakat does not have to be paid on property until a (lunar) year has elapsed over it." And Allah knows best!