I remember Papadopoulos tweeting about this cash six or eight months ago. He was theorizing that this is what was happening and that he felt the bills were somehow marked. He seemed to think they could be reversed tracked back to the FBI sting.
Mueller was even more devious. If Papadopoulos had taken the cash back to the US he could be charged with because it was 10,000.00 dollars. Over the limit to carry.