Revealing mistake: When Schultz checks the barbed wire to see if it is fixed now, he noticeably wiggles the vertical wire, but only barely touches the horizontal one - because it is rigged to come apart, and will again if he handles it too roughly.
Revealing mistake: When the boys are working on the fence, when the camera is on Hogan, you can see a modern metal building in the distance.
Answer: It's a comedy, not a documentary.
stiiggy
Perhaps it was counterfeit. There are numerous episodes where they deal in counterfeit monies.