Abu Nadra reported,

Talq said, I asked Ibn Umar and Ibn Abbas about currency exchange, and neither saw any harm in it. Later, while I was sitting with Abu Saeed Khudri, I asked him the same question, and he replied, "Whatever is given in excess is usury."I objected to this, citing what the other two had said, but he responded, "I will tell you only what I heard from the Messenger of Allah ﷺ. The owner of his date palms came to him with a measure of fine dates, though the Prophet's ﷺ usual dates were of a different sort.The Prophet ﷺ asked him, 'Where did you get these?'The man said, 'I took two measures and traded them for this one measure, since the market price for the one is such-and-such, and for the other such-and-such.'The Messenger of Allah ﷺ said, 'Woe to you! You have committed usury. If you want that, sell your dates for some other commodity, then use that commodity to buy whatever dates you wish.'"Abu Saeed then said, "Which is more deserving to be called usury, dates for dates, or silver for silver?"The narrator said, I went back to Ibn Umar afterward, and he forbade me from it. I did not go to Ibn Abbas. But Abu Sahba told me that he had asked Ibn Abbas about it in Mecca, and he disapproved of it.

عن أبي نضرة قال

سألت ابن عمر وابن عباس عن الصرف، فلم بريا به بأسا، فإني لقاعد عند أبي سعيد الخدري، فسألته عن الصرف، فقال: ما زاد فهو ربا فأنكرت ذلك لقولهما، فقال: لا أحدثك إلا ما سمعت رسول الله ﷺ، جاءه صاحب نخله بصاع من تمر طيب، وكان تمر النبي ﷺ هذا اللون. فقال له النبي ﷺ: أني لك هذا؟ قال: انطلقت بصاعين، فاشتريت به هذا الصاع، فإن سعر في السوق كذا، وسعر هذا كذا. فقال رسول الله ﷺ: ويلك أربيت! إذا أردت ذلك فبع تمرك بسلعة، ثم اشتر بسلعتك أي تمر شئت. قال أبو سعيد: فالتمر بالتمر أحق أن يكون ربا أم الفضة بالفضة؟ قال فأتيت ابن عمر بعد، فنهاني، ولم آت ابن عباس قال: فحدثني أبو الصهباء أنه سأل ابن عباس عنه بمكة، فكرهه.