Sahih Al Bukhari Hadith Number 2302

Narrated Abu Sa`id Al-Khudri and Abu Huraira: Allah’s Messenger (peace be upon him) employed someone as a governor at Khaibar. When the man came to Medina, he brought with him dates called Janib. The Prophet (peace be upon him) 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 (peace be upon him) 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).

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Hadith number 2302
Volume no 3
Chapter: 3 (To deputize one in exchanging money and weighing goods)