Note that it is easy to sell the dollar in Sudan and you can sell at the central bank but the exchange rate is very low against what you can get in the black market so tourists and expatriates prefer to deal with the black market.
What if it's the other way around: can you sell your SDG back to the central bank for dollars before you leave the country? I guess you can't, otherwise the black market sellers would get rich.
Just a hypothetical question, I'm not going to Sudan (or anywhere else) as a tourist any time soon.