How can the effective cause of usury (riba) — measuring by volume or weight, along with edibility — be determined in the four categories, taking into consideration the changes in and exchange of methods of measuring by volume and weight, and the transformation of some weighed items to be sold by count or estimation?

The four imams differed regarding the 'illah (effective cause) of riba (usury/interest) in the explicitly mentioned categories. Among these views is that the 'illah is edibility (ta'm) combined with measurement by volume (kayl) or weight (wazn), which was the old opinion of Al-Shafi'i and a narration from Ahmad. They also differed concerning selling something typically measured by volume, by weight, and vice-versa. Some considered the legal text (nass shari'), others considered the custom (urf) of the people of Hijaz or custom in general, and some differentiated between what was originally measured by weight and what was originally measured by volume. As for what is not explicitly stipulated regarding its standard, it was said that it should be based on the customs of people, or that it should refer to the custom in Hijaz. If it has no custom in Hijaz, then either its local custom should be considered, or it should be referred to the closest similar items in Hijaz. This disagreement supports the view of the Shafi'is that the 'illah of riba is edibility (ta'm) alone, because volumetric measurement (kayl) varies across different countries and times, and the 'illah of a ruling must be constant in every time and place.