I used to sell camels at al-Baqi for dinars and take dirhams for them, and sell for dirhams and take dinars for them. I would take these for these and give these for these. I went to the Messenger of Allah ﷺ who was in the house of Hafsah. I said: Messenger of Allah , take it easy, I shall ask you (a question): I sell camels at al-Baqi'. I sell (them) for dinars and take dirhams and I sell for dirhams and take dinars. I take these for these, and give these for these. The Messenger of Allah ﷺ then said: There is no harm in taking them at the current rate so long as you do not separate leaving something to be settled. (Using translation from Abū Dāʾūd 3354)   

كُنْتُ أَبِيعُ الْإِبِلَ فِي الْبَقِيعِ فَأَبِيعُ بِالدَّنَانِيرِ وَآخُذُ الدَّرَاهِمَ وَأَبِيعُ بِالدَّرَاهِمِ وَآخُذُ الدَّنَانِيرَ فَوَقَعَ فِي نَفْسِي مِنْ ذَلِكَ فَأَتَيْتُ رَسُولَ اللهِ ﷺ وَهُوَ فِي بَيْتِ حَفْصَةَ أَوْ قَالَ حِينَ خَرَجَ مِنْ بَيْتِ حَفْصَةَ فَقُلْتُ يَا رَسُولَ اللهِ رُوَيْدَكَ أَسْأَلُكَ إِنِّي أَبِيعُ الْإِبِلَ بِالْبَقِيعِ فَأَبِيعُ بِالدَّنَانِيرِ وَآخُذُ الدَّرَاهِمَ وَأَبِيعُ بِالدَّرَاهِمِ وَآخُذُ الدَّنَانِيرَ فَقَالَ لَا بَأْسَ أَنْ تَأْخُذَهَا بِسِعْرِ يَوْمِهَا مَا لَمْ تَفَرَّقَا وَبَيْنَكُمَا شَيْءٌ وَبِهَذَا الْمَعْنَى  

[Machine] I used to sell camels in al-Baqi' and people would bring me dirhams and I would sell them to a man for dinars, and he would give them to me tomorrow. So, I went to the Messenger of Allah ﷺ and asked him about this. He said, "If you sell something to a man for gold or silver, do not separate from him until the transaction is completed, and there is some kind of clothing between you." It is narrated in one of the two narrations from Isra'il from Simak and from Abu Al-Ahwas from Simak, and the hadith is unique in its elevation of Simak bin Harb [as the transmitter] from Sa'id bin Jubair among the companions of Ibn 'Umar.  

كُنْتُ أَبِيعُ الْإِبِلَ بِالْبَقِيعِ فَيَجْتَمِعُ عِنْدِي مِنَ الدَّرَاهِمِ فَأَبِيعُهَا مِنَ الرَّجُلِ بِالدَّنَانِيرِ وَيُعْطِينِيهَا لِلْغَدِ فَأَتَيْتُ رَسُولَ اللهِ ﷺ فَسَأَلْتُهُ عَنْ ذَلِكَ فَقَالَ إِذَا بَايَعْتَ الرَّجُلَ بِالذَّهَبِ وَالْفِضَّةِ فَلَا تُفَارِقْهُ وَبَيْنَكُمَا لُبْسٌ وَبِقَرِيبٍ مِنْ مَعْنَاهُ رُوِيَ فِي إِحْدَى الرِّوَايَتَيْنِ عَنْ إِسْرَائِيلَ عَنْ سِمَاكٍ وَعَنْ أَبِي الْأَحْوَصِ عَنْ سِمَاكٍ وَالْحَدِيثُ يَنْفَرِدُ بِرَفْعِهِ سِمَاكُ بْنُ حَرْبٍ عَنْ سَعِيدِ بْنِ جُبَيْرٍ مِنْ بَيْنِ أَصْحَابِ ابْنِ عُمَرَ