What is the ruling on selling dollar checks for less than their value to benefit from the price difference, especially since the bank only disburses the amount in Sudanese pounds and at an exchange rate lower than the black market rate?

Checks that may be sold for cash are those whose content is considered as having been received [qabdh], such as bank checks, certified checks, or similar instruments. As for non-bank checks or uncertified checks, their receipt is not considered as a constructive [hukmi] receipt of their content, and it is not permissible to deal with them in transactions where possession [qabdh] is a condition. Checks that cannot be cashed in the currency in which they were issued, but rather in the country's currency at the market rate, are not considered to have their content constructively received, and therefore, their sale is not valid.