Question: In this day and age the price of gold and silver is always fluctuating, so how will the Nisāb of Zakāh be calculated?
Answer: As soon as a person becomes the owner of an amount of money reaching the value of the Nisāb of Zakāh, then immediately his period of one year will begin, at the completion of which, he will have to pay his Fard Zakāh. If the gold and silver he has on hand does not reach the value of Nisāb then the value of the merchandise he has in his possession which he wants to sell will also be added to it, and if the total reaches the Nisāb of gold or silver then Zakāh will be Fard on him accordingly.
[Derived from the following books: Fatāwā ‘Alamgīrī, Shāmī and Aḥsanul Fatāwā]
Fatāwā Darul Uloom Zakariyyā, vol. 3, pg 100