What is the ruling concerning the sum of money taken by the questioner in exchange for his mediation in the sale of US dollars, given his knowledge that the other mediator increased the price without informing the original owner of the currency?

It appears that the questioner was acting as an agent for his friend in the currency exchange. All rights related to the contract revert to the principal, not the agent. Even if something was given as a gift to the agent, it is the right of his principal and is considered a reduction in price. Therefore, the questioner must return the difference he took to his friend, the owner of the money, or seek his forgiveness.