What is the ruling on the exchange of two different kinds of commodities where surplus is permitted, concerning delayed delivery (nasia')?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
For two different commodities where a surplus is permitted (e.g., gold for silver, wheat for dates), exchange must be 'hand to hand' (i.e., immediate receipt). There is no known difference of opinion regarding the permissibility of surplus when the exchange is immediate. The evidence is the Prophet's saying: 'If these things differ, then sell as you wish hand to hand,' and in another narration, 'Sell gold for silver as you wish hand to hand, but not on credit (nasia').' This immediacy is required because both items are subject to usury based on the same underlying reason (illah) for Ribawi prohibition, such as both being measurable by volume or weight.
Supporting text
If one of the exchanged items is a price (like dirhams or dinars) and the other is the commodity purchased, delayed exchange is permissible without dispute, as the concession in matters of advance payment (Salaf) necessitates this allowance for the primary capital (money). A dissenting view exists from Sa'id ibn Jubayr who held that if the utility of two items is very similar, surplus should not be permitted between them.