Is dealing in foreign currencies and buying them from the black market at a higher price instead of from the bank considered usury (riba) and forbidden (haram)?

It is not permissible to use the company's courier for private matters outside of the company without the company's permission. As for exchanging money in the black market, there is no harm in it. However, it is not permissible to take the exchange rate increase without the knowledge of your colleagues, because an agent (you) is not allowed to make a profit from his principals (your colleagues). Any increase gained by the agent must be returned to his principal, unless they specify a certain price for you and anything exceeding it is yours.