What is the ruling on the following cases regarding the exchange of dollars collected by a colleague, who then seeks the highest price, whether in the bank or on the black market: agreeing on a price with the buyer, then the buyer comes after some time to exchange the currencies? Or the buyer comes after some time to state that the price has increased, so the colleague sells at the agreed-upon price and takes the difference? Or the buyer comes after some time to state that the price has decreased, so the colleague sells at the agreed-upon price and pays the difference from his own money?

The exchange of currencies (sarf) is not permissible unless there is a mutual taking possession of the exchanged items in the same session of the contract. If one of your colleagues collects salaries to exchange them on behalf of their owners, this is an agency (wikala) for which they are allowed to request a fee. Any increase obtained by the agent belongs to the principal, not to the agent, unless the principal agrees otherwise, and any decrease is borne by the principal. Any bargaining and agreement on a specific price between the agent and the merchants is considered a non-binding promise. It is not permissible for the principals to agree with the agent that the agent guarantees against any decrease in exchange for profiting from an increase; rather, any increase belongs to the principals and any decrease is borne by them.