It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Do not sell one dinar for two dinars, one dirham for two dirhams, or one sa’ for two sa’s, for I fear you may fall into usury." A man stood up and said: "O Messenger of Allah! Tell me, if a man sells one horse for several horses, or a noble-bred camel for another camel, what is the ruling?" The Prophet (peace and blessings be upon him) said: "There is no harm, provided it is hand to hand."