Is it unlawful to set a currency exchange rate for a customer that is lower than the rate given by the merchant, when trading currency on behalf of another merchant?

It is not permissible for an agent to conceal the price difference from the merchant, as this constitutes deception and unlawful appropriation of his money. The increase in price belongs to the principal. The agent has the right to request a commission for his brokerage, but without trickery or deception.