Is money gained from currency exchange differences in the black market, instead of the bank rate, unlawful (haram)?

It is not permissible for the questioner to take the difference, because he is an agent for his friend, and an agent is entrusted and obligated to do what is best for his principal. The agent is bound by the terms of the agency, presents an account to the principal, and returns his money to him. Therefore, he must return this difference to his friend or seek his permission to keep it.