Must exchange between two similar commodities involve equality in measure or weight when Riba al-Fadl is applicable?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
When exchanging commodities belonging to the same genus, equality in measure or weight is obligatory when the effective cause (which necessitates equality) is present. If the items are of the same genus and meet the criteria for Riba al-Fadl (i.e., they are measured or weighed), the exchange must be measure for measure or weight for weight. This is demonstrated by the Hadith prohibiting unequal exchange of wheat for wheat, or barley for barley.
Supporting text
Sa'id ibn Jubayr held an alternative view stating that any two items whose utility is close, such as wheat and barley, or dates and raisins, should not be exchanged with excess, as they take the place of two varieties of the same genus. This view is rejected because it contradicts the permission granted by the Prophet to exchange gold for silver hand-to-hand regardless of the difference, despite their close utility.