unless it's just a one-off purchase, I feel like a much better strategy would be to pay in $20's and have a fraction of them be fake. if someone paid me with a (hypothetical) $500 bill, I damn sure take at least a minute to hold it up to the light and look for security features. they paid me with 25 $20 notes, I would probably just count them and call it a day. if 10-20% of them were fake, I might not realize until I tried to deposit them.