Musnad Ahmad 27:1110

Yazid narrated to us, Hammad ibn Salamah informed us, from Simak ibn Harb, from Sa'id ibn Jubayr, from Ibn Umar, who said: I used to sell camels at al-Baqi', and I would sell them for dinars and take dirhams in their place, and I would sell them for dirhams and take dinars in their place. So I came to the Prophet, peace and blessings of Allah be upon him, and I found him coming out of the house of Hafsah. I asked him about that, and he said: "There is no harm in it, according to the [current] value."