Dear Felix
As you know, we are happy to accommodate you with credit terms to better facilitate doing business together.
Unfortunately, our records indicate that your $[CREDIT LIMIT] credit limit has been exceeded. Since we know you to be a valued and conscientious customer, however, your purchase (P.O. #[PURCHASE ORDER]) was approved nonetheless. I urge to you check your records to determine whether our records are incorrect.
If you determine that our records are correct, I request that you forward to us payment in the amount of $[DIFFERENCE], to bring your account up to date. I apologize for any inconvenience this may have caused.
Regards
Lydia