Riba (usury) is one of the types of exploitation in transactions. It entails great harm and involves unlawful earnings and taking extra money wrongfully. Therefore, it has been prohibited in all the laws that have been revealed. Riba al-Fadl (usury of surplus) is one of the types of Riba, and it means selling an item that is subject to Riba rulings for another of the same type with excess in one of the two, like selling a Sā‘ of wheat for two Sā‘s thereof, or one hundred grams of gold for one hundred and ten grams thereof. In this Hadīth, Ma‘mar ibn ‘Abdullah ibn Nadlah (may Allah be pleased with him) narrates that he sent his servant with one Sā‘ of wheat and ordered him to sell it for cash, i.e., for dirhams and dinars, then buy barley with its price. The boy went and bought one Sā‘ of barley and part of a Sā in return for the wheat directly without selling the wheat first and then buying the barley with its price. Perhaps he did that because buying barley was the goal and it was achieved by this. When the servant returned to Ma‘mar, he informed him of what he had done, i.e., selling one Sā‘ of wheat for one Sā‘ and more of barley. Ma‘mar asked him: Why did you do that?! This is an objecting question. Then, he ordered him to return and bring back the wheat and return the barley and not to take the barley from him except like for like in terms of weight. Then, he clarified to him the reason for returning the extra barley, as he heard the Messenger of Allah (may Allah's peace and blessings be upon him) say: "Food for food," i.e., when selling one of them for another, it should be "like for like," i.e., in terms of weight and amount without any addition or surplus, and the two foods here refer to those of the same type, as it was mentioned in a version in Sahīh Muslim Collection that the Prophet (may Allah's peace and blessings be upon him) said: "If these classes differ, sell as you wish as long as payment is made hand to hand." Ma‘mar (may Allah be pleased with him) said: "Our food at that time was barley," i.e., it was the staple food during the time of the Prophet (may Allah's peace and blessings be upon him). Some of those who were present objected that barley and wheat are not of the same type, i.e., wheat and barley are two different types, therefore, surplus between them is not unlawful. Ma‘mar (may Allah be pleased with him) said: "I am afraid that it could be similar," i.e., a similar case. In other words, he was afraid it was included among the similar types and, thus, should have the same ruling of the prohibition of Riba since wheat and barley are close and each of them is called food. He (may Allah be pleased with him) did this out of religious prudence and as a precautionary measure. The Hadīth highlights the prohibition of Riba al-Fadl (usury of surplus). It shows how the Companions were cautious not to fall into committing what is unlawful..