One of the most common scam is note switching. For instance, you pass him a 500 peso note, he turns around and suddenly it becomes a 50 peso note! He then asks for more and you might find your yourself at a momentary loss as it totally came out of the blue and begin questioning yourself.
This is extremely common as both notes look similar (same digits, similar colours).
Another variation is for the merchant to turn around and come back with a torn note! So always, be wary of what you pass to the driver (can be any merchant in fact) and if possible, use small notes.

CHECK THIS OUT  Don’t Pay The Hitman