Selling two items that belong to the same genus in exchange for one another while adding—either to both or to one of them—something that is not of that genus, an arrangement the jurists illustrate with the example of “mudda ʿajwah wa dirham” (one measure of Ajwah dates and a dirham), falls into three categories:
• First: The aim is to trade a ribā-liable commodity for its own kind in unequal amounts, or to tack on to the smaller amount an item of a different genus as a loophole. The correct view is that this is categorically forbidden.
• Second: The aim is to trade a non-ribā-liable commodity—for example, a lactating ewe for another lactating ewe. The sound position is that this is permissible, the opinion of Mālik and al-Shāfiʿī.
• Third: Both components are intended, such as “a measure of Ajwah dates and a dirham for the like of them.” This is the well-known point of disagreement: Abū Ḥanīfah allows it, whereas Mālik, al-Shāfiʿī, and Aḥmad prohibit it.

It is not permissible to sell an unknown item until it has been distinguished, separated, and its individual units are identified.

Shaykh Muḥammad ibn Ibrāhīm said: “An invalid contract can under no circumstance become valid on its own. Anyone who wishes to rectify it must redo it in accordance with the recognized conditions.”
In this hadith the Prophet ﷺ did not treat the initial sale as valid; rather, he said, “It must not be sold until it has been separated.” Once the items have been separated, a new contract—distinct from the first—is concluded.