Abu Dawud Book 16, Hadith Number 3348.

Narated By Abdullah ibn Umar : 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 Apostle of Allah (PBUH) who was in the house of Hafsah. I said: Apostle 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 Apostle of Allah (PBUH) 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.