Imam Malik's Muwatta, Chapter: Business Transactions

Yahya related to me from Malik from Zayd ibn Aslam that Ata ibn Yasar said, "The Messenger of Allah, may Allah bless him and grant him peace, said, 'Dried dates for dried dates is like for like.' It was said to him, 'Your agent in Khaybar takes one sa for two.' The Messenger of Allah, may Allah bless him and grant him peace, said, 'all him to me.' So he was called for. The Messenger of Allah, may Allah bless him and grant him peace, asked, 'Do you take one sa for two?' He replied, 'Messenger of Allah! Why should they sell me good dates for assorted low quality dates, sa for sa!' The Messenger of Allah, may Allah bless him and grant him peace, said, 'Sell the assorted ones for dirhams, and then buy the good ones with those dirhams.' "