Squigly Squigly's Brain Teasers

Question:

An old man had been browsing for some time. He finally walked up to the girl behind the counter and handed her a book. She looked at the tag on the inside cover and said, "That's 4.50 please." The man handed her the money and promptly walked away without the book. The girl saw him leave empty-handed but she didn't call out to him in any way. Why?

Answer:

The old man was paying a fine for an overdue book in a library.