I had the same thing with gold earlier at Freeport today. Picked up 10 tons of gold in an USS. When arriving at Freeport I went to the Black market and tried to sell it. It didnt show up. But after reentering the black market it suddenly popped up and I could sell it. I think it was lagg which caused it. Many players at Freeport, in the end it just freezed when trying to SC out from Freeport. Dont dump the cargo just try and wait a bit and reenter the black market. Then its ok I suppose.