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).  

كُنَّا نُرْزَقُ تَمْرَ الْجَمْعِ وَهْوَ الْخِلْطُ مِنَ التَّمْرِ وَكُنَّا نَبِيعُ صَاعَيْنِ بِصَاعٍ فَقَالَ النَّبِيُّ ﷺ لاَ صَاعَيْنِ بِصَاعٍ وَلاَ دِرْهَمَيْنِ بِدِرْهَمٍ