What is the ruling on exchanging hard currency in the black market to obtain local currency, given that the rate is better and faster than banks and constantly changing?

There is no harm in selling currencies in the black market if the transactions are legitimate, meaning the exchange is hand-to-hand, based on the Prophet Muhammad's (peace be upon him) saying: "Gold for gold, silver for silver... hand-to-hand," and his saying regarding currency exchange: "If it is hand-to-hand, then there is no harm; but if it is deferred, then it is not permissible."