Unsure if this goes to bugs...but think its more of a scripting error.
The goblin at the harbor does not sell Topas powder when requested...instead he sells. you a cut topas.
I think it has to do with the way you spell it when you ask to buy it. Make sure you spell it exactly as it shows up on his menu when you ask him what he sells. I've bought topaz powder before, so I know it's possible.