Sahih al-Bukhari Book 34, Hadith 33
We used to be given mixed dates (from the booty) and used to sell (barter) two Sas of those dates) for
one Sa (of good dates). The Prophet (ﷺ) said (to us), "No (bartering of) two Sas for one Sa nor two
Dirhams for one Dirham is permissible", (as that is a kind of usury). (See Hadith No. 405).
Allah's Messenger (ﷺ) said, "The seller and the buyer have the right to keep or return goods as long as they have not parted or till they part; and if both the parties spoke the truth and described the defects and qualities (of the goods), then they would be blessed in their transaction, and if they told lies or hid something, then the blessings of their transaction would be lost."
An Ansari man, called Abu Shu'aib, came and told his butcher slave, "Prepare meals sufficient for five persons, for I want to invite the Prophet (ﷺ) along with four other persons as I saw signs of hunger on his face." Abu Shu'aib invited them and another person came along with them. The Prophet (ﷺ) said (to Abu Shu'aib), This man followed us, so if you allow him, he will join us, and if you want him to return, he will go back." Abu Shu'aib said, "No, I have allowed him (i.e. he, too, is welcomed to the meal).