Abu Ayyash (may Allah have mercy on him) said: I asked Sa'd bin Abi Waqqas (may Allah be pleased with him) about buying sult (a kind of barley) for dried dates. Sa'd (may Allah be pleased with him) said: "Is there a difference in value between the two?" They said: Yes. He said: "Then it is not right." Sa'd (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) was asked about buying fresh dates for dried dates, and the Messenger of Allah (peace and blessings be upon him) asked: "Is there a difference between them?" They said: Fresh dates decrease. The Messenger of Allah (peace and blessings be upon him) said: "Then it is not right."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10563