Sayyiduna Abu Hurayrah and Sayyiduna Abu Sa'id (may Allah be pleased with them both) both said that the Messenger of Allah (peace and blessings be upon him) appointed a man of Banu Adi of the Ansar as governor over Khaybar. He came with fine-quality dates, and he (peace and blessings be upon him) asked: "Are all the dates of Khaybar like this?" He said: No, O Messenger of Allah! We take one sa' (of these) for two sa's of mixed dates. The Prophet (peace and blessings be upon him) said: "Do not do that; rather (exchange) equal for equal, or sell these and buy those with their price. And the same applies to what is weighed."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10518