How can the 'illah (effective cause) of riba (usury/interest) in usurious commodities be controlled nowadays, especially with the change in their measurement methods (from volume to weight and vice versa, or selling by count), and with the existence of manufactured foods derived from them?

The jurists differed on the illah (underlying reason or effective cause) for riba in the six categories mentioned in the hadith of Ubadah ibn al-Samit. The most preponderant opinion is that the illah for gold and silver is their monetary value (thamanīyah), and for the other four categories, it is edibility (ṭuʿm) combined with being measurable by volume (kayl) or by weight (wazn). The criterion for something being measurable by volume, weight, or count is the sharʿī (Islamic legal) standard, meaning what was customary during the time of the Prophet, peace and blessings be upon him, as indicated by the hadith: "The weight is the weight of the people of Makkah, and the measure is the measure of the people of Madinah." Therefore, watermelon, for example, which is counted, is not subject to riba, even if it is sold by weight now. And whatever did not have a custom (urf) during the time of the Prophet, peace and blessings be upon him, is attached to the closest resembling item in Makkah and Madinah, or its local custom is considered for the ease of the people.

It is not permissible to sell a ribawi item raw for cooked, nor its origin for its juice (oil), due to the impossibility of achieving equality. Examples include selling wheat for bread, or sesame seeds for their oil. However, this is rare in modern transactions, as these items are typically purchased with money.