Fiqh-us-Sunnah - 3.20

Hadith 3.20.1

How is Zakah on Trade Money to be Paid?

One who possesses merchandise with a nisab for a year should pay zakah on it, the amount of which is a quarter of a tenth of its value. This should be done by a businessman every year. However, the period of a year does not come into effect unless his inventory constitutes a nisab.
Assuming a businessman possesses merchandise short of a nisab and part of a year has passed, his inventory subsequently increases through an unusual rise in value (because of supply and demand or through price fluctuation) so that it constitutes a nisab; or he sold merchandise for the price of a nisab; or during the course of the year he comes to possess other merchandise which, together with his previous amount, completes a nisab; then, the hawl (for the purpose of zakah) starts at that time, and the time elapsed is not taken into consideration. This is the view of the Hanafiyyah, ath-Thauri, ashShaf'i, Ishaq, Abu 'Ubaid, Abu Thaur, and Ibn al-Mundhir.
According to Abu Hanifah, if the merchandise in possession constitutes a nisab at the beginning of the year and also at the end, zakah will still be applicable even though the nisab might have decreased within that time. The reason is that it is difficult to ascertain its completeness in the intervening period.
The Hanbaliyyah hold that if the merchandise decreases during the course of the year and increases again until it constitutes a nisab, the (requisite) period of a year starts all over again because it has been interrupted in its course by the decrease.