Musnad Ahmad 938:81

Yazid ibn Harun narrated to us, Yahya ibn Sa'id informed us, from Nafi', from Ibn 'Umar, who said: Zayd ibn Thabit informed me: "That the Messenger of Allah (peace and blessings of Allah be upon him) granted a concession regarding 'Araya (the sale of fresh dates on the tree for dried dates of equal value), that it may be taken by the measure of its estimation in dates, so that its people may eat it fresh."