Cash Back Attack, Part 25

I am ringing up a lady’s purchase.

Customer: “I want to pay part with part cash, part card.”

Me: “Sure, no problem. Your total will be $27.68.”

She hands me a $20 bill, which I tender as cash, and then instructs her to swipe her card for the rest.

She then swipes her card and says:

Customer: “Oh, I need cash back.”

She then proceeds to select $20 cash back.

I just stand there, slack-jawed, staring at her for a moment before handing her the $20 bill that she handed me mere moments ago, which she snatches out of my hand in frustration after the realization hits.

There really is a collective brain cell that all customers share, huh?

Related:
Cash Back Attack, Part 24
Cash Back Attack, Part 23
Cash Back Attack, Part 22
Cash Back Attack, Part 21
Cash Back Attack, Part 20