Can't count

Ok I need some opinions here. I just came back from Wal-Mart and got very irritated with my cashier. This is not a trait that I tend to display. My total for my bill came to $15.07. I gave the cashier $15.12. By the way this was a cashier working in the Pharmacy department. He looked at me and started to give me some of my money back. I inquired what was he doing and he responded you gave me to much. I then started to count out the money and explain to him that I did not have the exact change but the change that I did give him was because I could get a nickel back not pennies. He then preceded to tell me yes but it was to much. I explained to ring it up in the register and then it will show how much change he was to give me. He then said S--- and took out the nickel but still looked very confused. My thoughts were here I am out of work and if I applied for a job here I would get I the same pay as him perhaps a little more. And the language where was the customer service? As I preceded out the door I went out of my element again and advised the manager who was standing there of the issue. He then told me they had several complaints on him. My thoughts were why is he still there? Put him out collecting carts in the 104 degree sun and then he will learn to count. Perhaps the most disturbing is that we are enabling him to due this by excepting the behavior he is exhibiting. I don't know why this irritated me so much. What are your thoughts?