Is currency trading halal?

In matters of the currency notes, a person can sell one type of it for another, like toman against dinar or dollar, on credit, and on condition to receive more. But if he sells dollar for dollar, expecting more, then that transaction should not be on credit; otherwise it will be void and haraam. For example, if a person gives 100 dollars cash, on a condition that after six months he should be given 110 dollars, that is void and haraam.