I helped a Thai restaurant owner in similar case about an year ago, they needed a bit of video processing of the CCTV footage. An European looking lady skillfully put a 100 baht banknote below a 1000 baht one, and then claimed she gave 2000 and blamed the waitress.
The owner told her to come the next day to get refund after they review the CCTV footage. She never came.... probably on the way out she saw there was a camera right above her table.
It was very skillful and quick banknote placement, I had to split the footage in frames to notice the different banknote colour.