I used to buy dates from one of the Jewish clans who were called Banu Qainuqaʿ, and sell them at a profit. News of that reached the Messenger of Allah ﷺ who said: ʿO ʿUthman, when you buy, take your dues with nothing extra, and when you sell, give (the other partyʿs) dues with nothing less.ʿ Moosa bin Wardan narrated from Saʿeed bin al-Musayyab from ‘Uthman bin ʿAffan ؓ ... and he mentioned a similar report.
يَقُولُ سَمِعْتُ عُثْمَانَ يَخْطُبُ عَلَى الْمِنْبَرِ وَهُوَ يَقُولُ كُنْتُ أَبْتَاعُ التَّمْرَ مِنْ بَطْنٍ مِنَ الْيَهُودِ يُقَالُ لَهُمْ بَنُو قَيْنُقَاعَ فَأَبِيعُهُ بِرِبْحٍ فَبَلَغَ ذَلِكَ رَسُولَ اللهِ ﷺ فَقَالَ يَا عُثْمَانُ إِذَا اشْتَرَيْتَ فَاكْتَلْ وَإِذَا بِعْتَ فَكِلْ