Dear …
RE: Overpayment on Account
Thank you for recent your payment of __($amount)__. Upon review of your account, however, we have discovered that this was an overpayment. The amount due on your account was only __($amount)__, thereby now leaving a credit of __($amount)__. Please find enclosed our check in the amount of __($amount )__ to clear this credit. We trust that you will find all to be in order and look forward to being of service to you again soon. Thank you for your valued business!
Yours truly,
S