Please explain this English joke?

A woman lost her wallet. As a result, the wallet was picked up by an honest little boy and returned.

The lady looked at her wallet and said, "Well, that's interesting. When I lost my wallet, there was a 20-yuan bill in it, but now it's 20 1-yuan bills."

The child immediately replied: "That's right, miss. Last time I found a lady's wallet, but she didn't have any change for payment."

In fact, it was the little boy who took the lady's wallet. The 20 yuan in his wallet was changed into change~because he was afraid that the lady would not pay him on the excuse of not having change~~