Tirmidhi

Explanation:
1:
Imam Tirmidhi has mentioned two opinions, and in addition, some scholars have also mentioned a third interpretation: that someone buys wheat from another for one dinar on a one-month promise, and when a month passes, he goes and demands the wheat from him. The seller then says, "The wheat that is due to you from me—sell it back to me for two sacks of wheat on a two-month promise." This is an example of two sales in one sale.

2:
And it is precisely this ignorance (jahalah) that is the reason for the sale not being permissible, even though, outwardly, the price of both seems to be determined.

English Translation:

One explanation of "two transactions in one sale" has been given by the author rahimahullah himself. Some discussion of this has already been presented under hadith 4615, namely that if an agreement is reached on a single transaction, whether on credit or cash, then there is no harm in the difference between the cash and credit price, because this difference is natural. However, if a single transaction is not finalized, for example: "I will sell you such-and-such item on the condition that you sell me such-and-such item," then this is not permissible, because by making the sale of the second item a condition, an unlawful benefit is being taken.

Allamah Ibn Qayyim rahimahullah has explained "two sales in one sale" as follows: (The seller says to the buyer:) "I will give you such-and-such item on credit for one hundred rupees, and I will take from you, right now, eighty rupees in cash." And if, instead of the item, he gives eighty rupees and after a year takes back one hundred rupees, it is clear that two transactions are taking place in one sale. And this is explicit usury (riba). Such a sale will be invalid (fasid), because in reality, this is not a sale at all. No item is being sold or purchased; rather, eighty rupees are being given and after a year, one hundred rupees are being taken in return, which is explicit usury. Even in the present era, some people do this. The word "sale" is used only to deceive. In such a case, he will only take back the eighty rupees. If he takes back one hundred rupees, then this will be usury. [فله أوكسَهُما أو الرِّبا] These last two scenarios are the best explanation of this hadith ("two transactions in one sale"), and both are prohibited. However, the first scenario, involving cash and credit, is valid. If the transaction is finalized in one form, then there can be a difference between the cash and credit price, because this is one sale, not two; therefore, this scenario is not the correct explanation of this hadith. If ambiguity remains and no other scenario is finalized, then it can be brought under the purview of this hadith.