You know akin to using a Krugerand or a £50 note to buy a penny sweet, oh there were so many people who used to do this to get change when I worked in catering.
The trick is this, when you get your red Maos the 100RMBs preferably out of a cash machine before you hand it over look at the serial number and keep it in SIGHT at all times. The trick is thus, they take the 100RMB, something happens to the side which makes you look away and you're given a fake 100RMB note back.
Somebody tried this on me in Beijing once in a Hutong but wanted to 'go to the back room' for change. She was really pissed off when I told her I wanted her to leave the 100RMB note on the counter and she made all sorts of excuses.