All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If the value of the trade goods reaches the Nisaab (i.e. minimum amount of wealth liable for the payment of zakat) by themselves or after adding other kinds of wealth, then they are liable for the payment of zakat. The Nisaab of merchandise is an amount of which the value equals eighty-five grams of gold. If one's wealth reaches this value after the lapse of a full lunar year, then he is obliged to pay the due zakat, which equals 2.5% of the total sum of his wealth counted for zakat. This Muslim woman should strive to find out the exact amount of capital owned and the wealth liable for zakat at the beginning of each year, calculate and pay the zakat for all previous years. She should strive to remember the wealth liable for zakat at the beginning of the first year, pay its due zakat and then do the same with the following years. If she is currently economically strained and cannot pay them right now, then she is obliged to pay them whenever she can.
Allaah Knows best.