Narrated Ubadah ibn as-Samit:
The Apostle of Allah (peace_be_upon_him) said: Gold is to be paid for with gold, raw and coined, silver with silver, raw and coined (in equal weight), wheat with wheat in equal measure, barley with barley in equal measure, dates with dates in equal measure, salt by salt with equal measure; if anyone gives more or asks more, he has dealt in usury. But there is no harm in selling gold for silver and silver (for gold), in unequal weight, payment being made on the spot. Do not sell them if they are to be paid for later. There is no harm in selling wheat for barley and barley (for wheat) in unequal measure, payment being made on the spot. If the payment is to be made later, then do not sell them.