[The Rationale for Prohibiting Selling Gold for Gold on Credit]

Answer—and success is from Allah: The rationale for permitting the borrowing of gold, silver, grain, dates, and the like, and then returning their equivalents, is that a loan is an act of kindness toward the borrower; the lender thereby deserves praise and commendation, and the borrower owes him thanks—unlike a sale, considered as sale, in which the seller does not merit praise or commendation, nor is the buyer obliged to thank him.
— The nature of sale, considered as sale, is the hope of benefit and gain resulting from exchange.
— Selling grain for its like on credit, or gold for its like on credit, contains no benefit or gain for the seller; he merits neither praise nor thanks. Rather, by that sale he is a loser, deprived of benefiting from his wealth for a period.
There is no doubt that reason deems ugly any act that yields no benefit or interest—and all the more ugly is the act that yields pure harm with no benefit. The rulings of the Sharia have come in support of the rulings of sound reason.
By what we have mentioned, the difference between sale and loan is known.
— It is permissible to sell gold for its like immediately, and likewise grain for grain, dates for dates, etc., immediately—because each of the seller and buyer may desire what the other has, and that desire impels them to exchange; neither suffers loss in that sale.
— It is permissible to sell wheat for twice its amount of barley, hand-to-hand, because wheat is one genus and barley is another, and one of them is of higher value than the other.
— That is prohibited on credit, because the buyer who purchases wheat for twice its amount in barley on credit might grow lax in the increase and add more than the value of the wheat on account of the deferment; likewise, the seller might grow lax in extending the term on account of the increase in price. The All-Wise Lawgiver thus forbade that type of sale because of what we have mentioned of the risk of falling into ribā.
— As for selling it hand-to-hand, there is no laxity from the buyer in increasing the price without counter-value; thus it is not a setting in which ribā is likely.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2