Sunan an-Nasa'i 4582
It was narrated that Ibn 'Umar said:
"I used to sell camels at Al-Baqi and I would sell Dinars in exchange for Dirhams. I came to the Prophet in the house of Hafsah and said: 'O Messenger of Allah, I want to ask you:  I sell camels in Al-Baqi and I sell Dinars in exchange for Dirhams. He said: 'There is nothing wrong with it if you take the price on that day, unless you depart when there is still unfinished business between you both (buyer and seller)."'
أَخْبَرَنِي أَحْمَدُ بْنُ يَحْيَى، عَنْ أَبِي نُعَيْمٍ، قَالَ حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ سِمَاكِ بْنِ حَرْبٍ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عُمَرَ، قَالَ كُنْتُ أَبِيعُ الإِبِلَ بِالْبَقِيعِ فَأَبِيعُ بِالدَّنَانِيرِ وَآخُذُ الدَّرَاهِمَ فَأَتَيْتُ النَّبِيَّ صلى الله عليه وسلم فِي بَيْتِ حَفْصَةَ فَقُلْتُ يَا رَسُولَ اللَّهِ إِنِّي أُرِيدُ أَنْ أَسْأَلَكَ إِنِّي أَبِيعُ الإِبِلَ بِالْبَقِيعِ فَأَبِيعُ بِالدَّنَانِيرِ وَآخُذُ الدَّرَاهِمَ قَالَ ‏
"‏ لاَ بَأْسَ أَنْ تَأْخُذَهَا بِسِعْرِ يَوْمِهَا مَا لَمْ تَفْتَرِقَا وَبَيْنَكُمَا شَىْءٌ ‏"‏ ‏.‏