I asked Ibn Abbas (Allah be pleased with them) about the conversion (of gold and silver for silver and gold). We said: Is it hand to hand exchange? I said: Yes. whereupon he said: There is no harm in it. I informed Abu Sa'id about it, telling him that I had asked Ibn 'Abbas about it and he said: Is it hand to hand exchange? I said: Yes, whereupon he said: There is no harm in it. He (the narrator) said, or he said like it: We will soon write to him, and he will not give you this fatwa (religious verdict). He said: By Allah, someone of the boy-servants of Messenger of Allah ﷺ brought dates, but he refused to accept them (on the plea) that those did not seem to be of the dates of our land. He said: Something had happened to the dates of our land, or our dates. So I got these dates (in exchange by giving) excess (of the dates of our land), whereupon he said: You made an addition for getting the fine dates (in exchange) which tantamounts, to interest; don't do that (in future). Whenever you find some doubt (as regards the deteriorating quality of) your dates, sell them, and then buy the dates that you like. (Using translation from Muslim 1594c)  

أَوَقَالَ ذَاكَ؟ أَمَّا إِنَّا سَنَكْتُبُ إِلَيْهِ فَلَنْ يُفْتِيَكُمُوهُ قَالَ فَوَاللهِ لَقَدْ جَاءَ بَعْضُ فِتْيَانِ رَسُولِ اللهِ ﷺ بِتَمْرٍ فَأَنْكَرَهُ فَقَالَ كَأَنَّ هَذَا لَيْسَ مِنْ تَمْرِ أَرْضِنَا؟ فَقَالَ كَانَ فِي تَمْرِنَا الْعَامَ بَعْضُ الشَّيْءِ وَأَخَذْتُ هَذَا وَزِدْتُ بَعْضَ الزِّيَادَةِ فَقَالَ أَضْعَفْتَ أَرْبَيْتَ لَا تَقْرَبَنَّ هَذَا إِذَا رَابَكَ مِنْ تَمْرِكَ شَيْءٌ فَبِعْهُ ثُمَّ اشْتَرِ الَّذِي تُرِيدُ مِنَ التَّمْرِ