The current rate is what counts in zakat

Question

A bookseller, for example, printed or bought ten thousand books at ten rupees per hundred, for one thousand rupees. Then, having fixed a profit of four rupees per hundred, he began to sell them. In the course of the year five thousand books (whose original cost is five hundred rupees) were sold, on which, at four rupees per hundred, he earned a profit of two hundred rupees, and he has a stock of five thousand books with him.

Answer

At the end of the year, zakat is to be paid on whatever value the goods on hand have at the market rate (not according to the original purchase cost). It is better that, at this same rate, he give one fortieth in the form of the books themselves as zakat, so that the proportion between the principal goods and the zakat is maintained with respect to the rate.

Source: Fatawa Mahmudiyya

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