This is really getting to me. I think you have confused the FBI with the Secret Service.
The FBI is part of the Justice Department, not the Department of the Treasury. Of course, the FBI would want in on this investigation. It would be because they investigate organized crime through RICO. So their role makes sense, but not particularly because money is involved and not because of the Treasury Department.
The US Secret Service does investigate counterfiting and major fraud, but I am not sure the crimes involved here would rise to that level.
I also have to admit that I am shocked that Lois did not mess up and call Mr. Olsen "Jimmy".
