If a person sells a commodity that is sold in metres or yards, such as cloth, or a commodity that is sold by count, such as eggs and walnuts, and he takes more in return, there is no problem except if both [the commodity being sold and the payment in exchange (ʿiwaḍ)] are of the same commodity and the transaction has a period, in which case its validity is problematic (maḥall al‑ishkāl) [i.e.
based on obligatory precaution, it is not valid].[3] An example [of such a problematic transaction] is when a person gives ten walnuts at present to receive twelve walnuts after one month. The same applies to selling currency. Therefore, there is no problem if, for example, a person sells British pounds sterling for another currency such as dinars or dollars, whether that be at present or at another time.
However, if the person wishes to sell some currency for the same currency and receive more in return, then that transaction must not have a period; otherwise, its validity is problematic [i.e. based on obligatory precaution, it is not valid]. An example [of such a problematic transaction] is when a person sells £100 at present to receive £110 after six months.