Sahih al-Bukhari Book 34, Hadith 34
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).
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).
The Prophet (ﷺ) aid, "The buyer and the seller have the option to cancel or to confirm the deal, as long as they have not parted or till they part, and if they spoke the truth and told each other the defects of the things, then blessings would be in their deal, and if they hid something and told lies, the blessing of the deal would be lost."