If a person sells by way of a prepayment transaction a commodity made of gold or silver and accepts gold or silver money in exchange, the transaction is invalid. However, if a person sells a commodity or currency which is not made of gold or silver and takes another commodity or gold or silver money in exchange, the transaction is valid as per the details that will be mentioned in the seventh condition in the next ruling.
And the recommended precaution is that in exchange for the commodity one sells, he should receive money, not another commodity.