Does Riba al-Fadl apply only to the six commodities explicitly mentioned in the Hadith of 'Ubadah ibn as-Samit?

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The prohibition of Riba al-Fadl applies to all items that share the same effective cause ('illah) as the specified commodities. The most prevalent narration from Ahmad establishes that the 'illah for gold and silver is being a weighted medium of exchange, and the 'illah for the four foodstuffs is being a measured staple/foodstuff. Consequently, Riba applies to every item measured by weight or measure of its own kind, whether it is consumed or not, such as grains, cotton, iron, and copper. This is supported by the Hadith specifying measure for measure for the six items and the narration from 'Ammar: "Usury is only in delay, except what is measured or weighed."