Is it permissible to take the exchange rate difference when converting clients' dollars, knowing that a better rate than the market rate can be obtained?

The agent is not allowed to take the exchange rate difference for himself unless he has the principal's permission. The increase belongs to the principal, not to the agent. The author of "Al-Kafaf" said: "And if it increases, the increase is for the principal, not for his agent who did not act justly." However, there is no harm for the agent to take the difference if the principal permits him, even if that is his wage. Ibn Abbas said: "There is no harm in saying, 'Sell this garment, and whatever exceeds such and such is yours.'"