Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) held that excess (tafadul) in exchange (sarf) was permissible and saw no harm in it. When he met Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him), Sayyiduna Abu Sa'id (may Allah be pleased with him) said: O Ibn Abbas! Do you not fear Allah? How long will you go on making people consume usury? Has it not reached you that one day the Prophet (peace and blessings be upon him) was with his wife Sayyidah Umm Salamah (may Allah be pleased with her) and he (peace and blessings be upon him) said: "I would love to eat Ajwah dates." So Sayyidah Umm Salamah (may Allah be pleased with her) sent two sa's of old dates to the house of a man of the Ansar, and in exchange she received one sa' of Ajwah dates. She presented them to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) liked them very much. He (peace and blessings be upon him) picked up a date, then stopped. He (peace and blessings be upon him) asked: "Where did you get these?" Sayyidah Umm Salamah (may Allah be pleased with her) said: I sent two sa's of old dates to so-and-so's house, and this one sa' came in exchange for them. He (peace and blessings be upon him) threw the date from his hand and said: "Return it, return it; I have no need of it. Dates for dates, wheat for wheat, barley for barley, gold for gold, silver for silver — it must be equal for equal and hand to hand; no increase or decrease in it is right. Whoever gives more or less has taken usury — (and so it is with) everything that is weighed or measured." Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: O Abu Sa'id! You have reminded me of a matter that I had forgotten; I seek Allah's forgiveness and repent to Him. After that he used to forbid it most strictly.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10519