It is well established that gold cannot be cloned (or transfigurated) permanently and that Goblins can easily detect all forms of counterfeit money. However, it seems that nothing would stop a wizard from transfiguring a bunch of 100 dollar notes and using them to buy a few gold bars. Goblins don't seem to mind "Muggle" gold and it's not like they could tell it was obtained illegally.
So, is there anything in the Harry Potter books or films which mentions why wizards couldn't do this?