Sayyiduna Abu Sa'id (may Allah be pleased with him) said: In the time of the Messenger of Allah (peace and blessings be upon him) we were given mixed dates as provision, that is, they were of mixed kinds, so we would take one sa' in exchange for two sa'. The Prophet (peace and blessings be upon him) said: "No. Nor one dirham for two dirhams."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10544