Is it permissible for a person to exchange one hundred dollars for thirty pounds per dollar with a first party, then sell the dollars to a second party for 30.25 pounds per dollar, and take the difference as commission?

This transaction is prohibited because it is a deferred usury (riba nasi'ah). Currencies, when exchanged one for another, require immediate possession (taqabud) at the time of the contract, based on the hadith of Ubadah ibn as-Samit, may Allah be pleased with him, that the Prophet, peace and blessings be upon him, said: "Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt – like for like, equal for equal, hand to hand. If these types differ, then sell as you wish, provided it is hand to hand." Narrated by Muslim. The decision of the "Fiqh Council of the Muslim World League" in Mecca states that paper currency is a distinct medium of exchange, having the same ruling as gold and silver. Therefore, usury applies to it in both its forms: excess (fadl) and deferment (nasi'ah).

Sheikh Ibn Baz, may Allah have mercy on him, was asked about such a transaction and he replied: "Dealing in buying and selling with currency is permissible, but on condition of immediate possession (taqabud) hand to hand if the currencies are different... As for it being deferred (to a later time), it is not permissible. Likewise, if immediate possession does not occur at the session, it is not permissible, because in such a situation, it is considered a type of usurious transaction. Therefore, immediate possession hand to hand at the session is necessary if the currencies are different." The solution to this is for one of the parties to agree with the other party to act as an agent for them in exchanging the currency for a specific agreed-upon amount, with any excess belonging to the agent.