It was narrated that Ibn 'Umar said: "I used to sell camels, and I used to buy gold for silver and silver for gold, Dinar for Dirham and Dirham for Dinar. I asked the Prophet P.B.U.H about that,and he said: 'I will you take one of them and give the other, then you and your companion should not separate until everything is clear (i.e., the exchange is completed):" (Hasan)