Is it permissible to sell inferior dates for one dirham and then immediately use that dirham to purchase superior dates (Junaib) from the same person?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
It is permissible if there is no prior agreement or subterfuge involved. This ruling is supported by the general principle that a sale of genus for another genus without precondition or prior agreement is valid, similar to selling to another party. Furthermore, transactions permitted once are permitted generally, like other sales. The evidence against this practice being strictly prohibited without intent is found in the Hadith where Bilal offered inferior dates for superior ones to the Prophet (PBUH) and was told, "Oh, that is the essence of usury (Riba), do not do that. But if you wish to purchase, sell the dates for another sale, then buy with it." Another narration regarding the governor of Khaybar states the Prophet (PBUH) explicitly commanded, "Do not do that. Sell the dates for dirhams, then buy Junaib dates with the dirhams." Crucially, the Prophet (PBUH) did not instruct the governor to sell the dates to someone else, implying that the mere sequence of transactions is not prohibited if the underlying exchange is sound.
Supporting text
A dissenting view from Ibn Abi Musa states it is impermissible unless the person goes to another party to purchase. Imam Ahmad, in one narration, preferred that the sale be made to another party, suggesting it is cleaner and ensures fulfillment of the equivalent value (gold in the case of currency exchange) without implying that the original seller might cheat on weight or measure upon the return of the value. However, the apparent conclusion is that this preference is based on recommendation (Istihbab), not obligation, especially if the person does not mind buying from either party.