Response to improper billing after payment

On [date] I received your letter which indicated that I had
a balance owing in the amount of $[dollar amount]. On
[date] I mailed a check to you in the amount of $[dollar amount].
You should have received my check prior to writing your
letter and I am, therefore, somewhat concerned.

If you have not received my check by the time this letter
arrives, I would appreciate hearing from you so that I may
issue a stop payment on the funds. I am waiting anxiously
to hear from you to get this matter quickly resolved.