What is the ruling on dealing in the sale of dollars via bank transfer in exchange for Sudanese pounds, either in cash or by transfer to an account, given the price differences and the scarcity of liquidity?

Paying the Dirham to the merchant and taking the Guinea from him, whether directly or through a deposit, is a currency exchange contract (Sarf). The only condition for it is immediate possession (al-taqabud), as long as the two currencies are different, and it is permissible to have a difference in their value. Possession can be actual (Haqiqi) or constructive (Hukmi), such as depositing into an account. So, if the deposit or hand-to-hand delivery occurs in the same session of the contract, the transaction is valid.