Messenger of Allah ﷺ employed someone as a governor at Khaibar. When the man came to Medina, he brought with him dates called Janib. The Prophet ﷺ asked him, "Are all the dates of Khaibar of this kind?" The man replied, "(No), we exchange two Sa's of bad dates for one Sa of this kind of dates (i.e. Janib), or exchange three Sa's for two." On that, the Prophet ﷺ said, "Don't do so, as it is a kind of usury (Riba) but sell the dates of inferior quality for money, and then buy Janib with the money". The Prophet said the same thing about dates sold by weight. (See Hadith No. 506).  

عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ وَأَبِي هُرَيْرَةَ ؓ أَنَّ رَسُولَ اللَّهِ ﷺ اسْتَعْمَلَ رَجُلاً عَلَى خَيْبَرَ فَجَاءَهُمْ بِتَمْرٍ جَنِيبٍ فَقَالَ أَكُلُّ تَمْرِ خَيْبَرَ هَكَذَا فَقَالَ إِنَّا لَنَأْخُذُ الصَّاعَ مِنْ هَذَا بِالصَّاعَيْنِ وَالصَّاعَيْنِ بِالثَّلاَثَةِ فَقَالَ لاَ تَفْعَلْ بِعِ الْجَمْعَ بِالدَّرَاهِمِ ثُمَّ ابْتَعْ بِالدَّرَاهِمِ جَنِيبًا وَقَالَ فِي الْمِيزَانِ مِثْلَ ذَلِكَ