I asked Al-Bara' bin ʿAzib and Zaid bin Arqam about money exchanges. Each of them said, "This is better than I," and both of them said, "Messenger of Allah ﷺ forbade the selling of silver for gold on credit. "  

عَنِ الصَّرْفِ فَكُلُّ وَاحِدٍ مِنْهُمَا يَقُولُ هَذَا خَيْرٌ مِنِّي فَكِلاَهُمَا يَقُولُ نَهَى رَسُولُ اللَّهِ ﷺ عَنْ بَيْعِ الذَّهَبِ بِالْوَرِقِ دَيْنًا  

that his father said, "The Prophet ﷺ forbade the selling of gold for gold and silver for silver except if they are equivalent in weight, and allowed us to sell gold for silver and vice versa as we wished."  

نَهَى النَّبِيُّ ﷺ عَنِ الْفِضَّةِ بِالْفِضَّةِ وَالذَّهَبِ بِالذَّهَبِ إِلاَّ سَوَاءً بِسَوَاءٍ وَأَمَرَنَا أَنْ نَبْتَاعَ الذَّهَبَ بِالْفِضَّةِ كَيْفَ شِئْنَا وَالْفِضَّةَ بِالذَّهَبِ كَيْفَ شِئْنَا