How is the monetary compensation (Arsh) for a defect calculated?

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The Arsh is calculated by determining the price the item would fetch if sound, and then determining its price when defective. The proportion of the difference between these two values relative to the sound price is applied to the original purchase price to determine the compensation due. For example, if an item worth ten is defective and is valued at nine, and the purchase price was fifteen, the defect reduced its value by one-tenth of its worth; therefore, the buyer is compensated one-tenth of the price, which is one and a half. This principle is supported by what Al-Kharaqi stated regarding taking the difference between soundness and defect, and Al-Hasan Al-Basri stated that the return should be based on the value of the defect in the price on the day of purchase, which Ahmad considered the best opinion heard.