Wow.
If you've worked a counter for any amount of time you have encountered this scam.
Which is why you leave the money out, like on top of the money drawer, where everybody can see it and you say what the bill is and how much change is due.
Once the person accepts the change then you put the bill away.
I had so many people (scum) try this sort of thing that it got to be funny.
Some half-wit would buy a coke and pay with a 20. His change would come out to be 19.25 and then once handed the money the idiot would say "I gave you a hunnerd."
"No sir, you did not, see that twenty right there that was yours."
"I gave you a hunnerd."
"Get the F! out of my store, loser!"
"I gave you a hunnerd."
"I'm getting the bat."
(As he runs away) "I gave you a hunnerd."
And natch when I checked my till later, I was perfect.
This crack-head grifter scumbags thought they could get away with it. Now to see a cop doing it makes me laugh even harder.