If you didn't have enough money to buy an item in a shop the shop assistanct gave you the shortfall and this was the change.
A childhood belief about money

others believed things like this
- that people in shops gave you 'change' if you didn't quite have enough money to pay.— suzy
- I used to believe it was illegal to shop at Goodwill unless you were poor.— Aretia
- I used to think that a strip mall was where you shop naked.— WD
- I always thought that shoplifting is about lifting the shop from the ground.— Helen
- I used to think testers were short for testacles! So in the shop i used to say can i try a testacle!— Ro
- I use to believe that a stripmall was a place where people could shop nakedAnon