Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) "forbade muzabanah", and muzabanah is that a man sells fresh dates by measure in exchange for dry dates, and sells grapes in exchange for a measured amount.
(b) In the narration of al-Shafi'i (may Allah have mercy on him): Muzabanah is selling the fresh dates of the trees in exchange for measured dry dates, and selling fresh grapes in exchange for measured raisins.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10635