Is zakat due on the value that has been in possession for a year, or on the current value?

If a Muslim owns a trade asset that reaches the Nisab (minimum threshold) and grows over the year to equal the original Nisab value, then he must pay Zakat on the entire amount when the original year is completed. This is by agreement of the majority of scholars. This is because the 'Hawl' (one year period) of the growth follows the 'Hawl' of the principal. Even if the value of the purchased commodity increases to the Nisab at the beginning of the 'Hawl', he adds the profit and pays Zakat on the total.