Sunan Ibn Majah, Chapter: The Chapters on Business Transactions

The Prophet used to give us dates from the collection (mixed) dates, and we would exchange them for dates that were better, and we add to the price. The Messenger of Allah said: "It is not right to give one Sa of dates for two Sa, nor one Dirham for two Dirham. A Dirham for a Dirham and a Dinar for a Dinar is allowed: the only difference between them is in weight (i.e., the weight must be equal.)" Sahih