In order to believe their story you have to believe that the delivery guy, for some reason, acted like a jerk and walked out with $7 dollars in change that they expected back then drove back with the change and acted humbly (if a little disgruntled). But apparently they didn't demand the change when he was there the first time and he thought it was a tip when he left with it.
So what would he have been acting like a jerk about if he thought they gave him a $7 dollar tip and they didn't ask for change at that time?
On top of that the manager gives out the first name of the girl he fired. That's class.