What is the religious ruling on trading foreign currencies in the black market without adhering to the official exchange rate, and on the wealth resulting from this trade?

It is permissible to trade currencies, provided that possession is taken at the contract session. If the currencies differ, it is permissible to have a difference in amount. However, if the currencies are the same, it is not permissible to have a difference in amount; rather, equality and possession at the contract session are required. There is no difference in this regard between the regulated market and the black market.