What is the effective cause ('illah) for applying Riba to gold and silver, and to the other four commodities (wheat, dates, barley, salt)?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
The dominant view, narrated in three reports from Ahmad, is that the effective cause ('illah) for Riba in gold and silver is their status as a weighted medium of exchange (mowzun). The 'illah for the four foodstuffs is their status as a measured staple/foodstuff (makil). Therefore, Riba applies to every item measured by weight or measure of the same kind, regardless of consumption status.
Supporting text
A second narration suggests the 'illah for the monetary items is their nature as currency (thaman), and for others, it is being a staple foodstuff ('itaam), thus restricting it only to consumed items. A third narration states the 'illah for non-precious metals is being a measured or weighed foodstuff, excluding fruits and non-edible items like saffron or iron, based on the narration: "There is no usury except in what is measured or weighed of what is eaten or drunk."