I asked Ibn Umar and Ibn Abbas (Allah be pleased with them) about the conversion of gold with gold but they did not find any harm in that. I was sitting in the company of Abd Sa'id al-Khudri (Allah be pleased with him) and asked him about this exchange, and he said: Whatever is addition is an' interest. I refused to accept it on account of their statement (statement of Ibn 'Abbas and Ibn 'Umar). He said: I am not narrating to you except what I heard from Messenger of Allah ﷺ. There came to him the owner of a date-palm with one sa' of fine dates, and the dates of Allah's Apostle ﷺ were of that colour. Allah's Apostle ﷺ said to him: Where did you get these dates? I went with two sa's of (inferior dates) and bought one sa' of (these fine dates), for that is the prevailing price (of inferior dates) in the market and that is the price (of the fine quality of dates in the market), whereupon Messenger of Allah ﷺ said: Woe be upon you! You have dealt in interest, when you decide to do it (i. e. exchange superior quality of dates for inferior quality) ; so you should sell your dates for another commodity (or currency) and then with the help of that commodity buy the dates you like. Abu Sa'ad said: When dates are exchanged for dates (with different qualities) there is the possibility (of the element of) interest (creeping into that) or when gold is exchanged for gold having different qualities. I subsequently came to Ibn 'Umar and he forbade me (to do it), but I did not come to Ibn 'Abbas; (Allah be pleased with them). He (the narrator) said: Abu as-Sahba' narrated to me: He asked Ibn Abbas (Allah be pleased with them) in Mecca, and he too disapproved of it. (Using translation from Muslim 1594d)   

عَنِ الصَّرْفِ فَلَمْ يَرَيَا بِهِ بَأْسًا وَإِنِّي لَقَاعِدٌ عِنْدَ أَبِي سَعِيدٍ الْخُدْرِيِّ فَسَأَلْتُهُ عَنِ الصَّرْفِ فَقَالَ مَا زَادَ فَهُوَ رِبًا فَأَنْكَرْتُ ذَلِكَ لِقَوْلِهِمَا فَقَالَ لَا أُحَدِّثُكُمْ إِلَّا مَا سَمِعْتُ مِنْ رَسُولِ اللهِ ﷺ جَاءَهُ صَاحِبُ نَخْلَةٍ بِصَاعٍ مِنْ تَمْرٍ طَيِّبٍ وَكَانَ تَمْرُ النَّبِيِّ ﷺ هُوَ الدُّونَ فَقَالَ لَهُ النَّبِيُّ ﷺ أَنَّى لَكَ هَذَا؟ قَالَ انْطَلَقْتُ بِصَاعِي وَاشْتَرَيْتُ بِهِ هَذَا الصَّاعَ فَإِنَّ سِعْرَ هَذَا بِالسُّوقِ كَذَا وَسِعْرَ هَذَا بِالسُّوقِ كَذَا فَقَالَ لَهُ رَسُولُ اللهِ ﷺ أَرْبَيْتَ إِذَا أَرَدْتَ ذَلِكَ فَبِعْ تَمْرَكَ بِسِلْعَةٍ ثُمَّ اشْتَرِ بِسِلْعَتِكَ أِيَّ تَمْرٍ شِئْتَ فَقَالَ أَبُو سَعِيدٍ فَالتَّمْرُ بِالتَّمْرِ أَحَقُّ أَنْ يَكُونَ رِبًا وَالْفِضَّةُ بِالْفِضَّةِ قَالَ فَأَتَيْتُ ابْنَ عُمَرَ بَعْدُ فَنَهَانِي وَلَمْ آتِ ابْنَ عَبَّاسٍ قَالَ فَحَدَّثَنِي أَبُو الصَّهْبَاءِ أَنَّهُ سَأَلَ ابْنَ عَبَّاسٍ عَنْهُ فَكَرِهَهُ