What if the dentist say you haven't paid him?

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I went to the dentist this morning to have my final finish. When I paid him the final installment, he told me I had not paid him anything earlier,unless I showed him the receipt right away, which I did not take with me. After a short time, the nurse found out I did pay part of the fees. I felt very angry. That should not be the same dentist I have known for over ten years. But after a second thought, I realized he might be too old to care about things beyond medical care. His age might have failed him to have a perfect memory. Anyway, tooth fixing is my focus. I felt sorry for him and could only say to him "Take care", instead of "Good bye".