Sayyiduna Ibn Umar (may Allah be pleased with them both) said: I used to sell camels at al-Baqi'. I would sell for dinars but take dirhams, and sell for dirhams but receive dinars. A doubt about this arose in my heart. I came to the Messenger of Allah (peace and blessings be upon him) while he (peace and blessings be upon him) was in the house of Sayyidah Hafsah (may Allah be pleased with her) — or he said: when he (peace and blessings be upon him) came out of the house of Sayyidah Hafsah (may Allah be pleased with her) — and I said: O Messenger of Allah (peace and blessings be upon him)! Wait, I want to ask you something. I sell camels at al-Baqi'; I sell for dinars and receive dirhams, and I sell for dirhams but receive dinars. He (peace and blessings be upon him) said: "There is no harm, if it is at that day's rate, as long as you two do not part while something remains (outstanding) between you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10513
Sa'id bin Jubayr (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both): I used to sell camels at al-Baqi', and dirhams would accumulate with me. I would sell them to a man for dinars, and he would give them to me the next morning. I spoke to the Messenger of Allah (peace and blessings be upon him) about this, and he (peace and blessings be upon him) said: "When you trade with a man in gold and silver, do not part from him while there is some matter or ambiguity (outstanding) between you."
Among the students of Ibn Umar, Simak bin Harb, from Sa'id bin Jubayr, is alone in narrating this hadith as marfu' (from the Prophet).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10514