Zaid, Abu 'Ayyash, the freed slave of Bani Zuhrah, told him that he asked Sa'd bin Abu Waqqas about buying wheat with barley. Sa'd said to him: "Which of them is better?" He said: "Wheat." He told him not to do that and said: "I heard the Messenger of Allah ﷺ being asked about buying fresh dates with dried dates, and he said: 'Do fresh dates decrease in weight when they become dry?' They said: 'Yes.' So he told them not to do that."  

أَنَّ زَيْدًا أَبَا عَيَّاشٍ مَوْلًى لِبَنِي زُهْرَةَ أَخْبَرَهُ أَنَّهُ سَأَلَ سَعْدَ بْنَ أَبِي وَقَّاصٍ عَنِ اشْتِرَاءِ الْبَيْضَاءِ بِالسُّلْتِ فَقَالَ لَهُ سَعْدٌ أَيَّتُهُمَا أَفْضَلُ قَالَ الْبَيْضَاءُ فَنَهَانِي عَنْهُ وَقَالَ إِنِّي سَمِعْتُ رَسُولَ اللَّهِ ﷺ سُئِلَ عَنِ اشْتِرَاءِ الرُّطَبِ بِالتَّمْرِ فَقَالَ أَيَنْقُصُ الرُّطَبُ إِذَا يَبِسَ قَالُوا نَعَمْ فَنَهَى عَنْ ذَلِكَ