Revealing mistake: When Le Beau holds up the gold painted brick and the "gold" brick, the ends reveal that they appear to be pieces of lumber. The ends seem to have grains consistent with lumber.
Revealing mistake: As Klink steps off the porch of his office, in the long shot you can tell it is his stunt double.
Answer: It's a comedy, not a documentary.
stiiggy
Perhaps it was counterfeit. There are numerous episodes where they deal in counterfeit monies.