The man in a hat

CUSTOMER:I bought two expensive dictionaries here half an hour ago, but I forgot to take them with me.
MANAGER:Who served you, sir?
CUSTOMER:The lady who is standing behind the counter.
MANAGER:Which books did you buy?
CUSTOMER:The books which are on the counter.
MANAGER:Did you serve this gentleman half an hour ago, Caroline? He says he's the man who bought these books.
CAROLINE:I can't remember. The man who I served was wearing a hat.
MANAGER:Have you got a hat, sir?
CUSTOMER:Yes, I have.
MANAGER:Would you put it on, please?
CUSTOMER:All right.
MANAGER:Is this the man that you served, Caroline?
CAROLINE:Yes. I recognize him now.