Excuses pour mauvais comportement d'un employé

Dear [Mr or Mrs or Sirs], We thank you for your letter of [date], in which you inform us of the regrettable conduct of our employee [nom]. We have immediately investigated the case and have informed the person concerned that we would not accept such another incident. We apologize for the situation and shall do everything necessary so that this will never happen again. Of course, we shall send you another employee to finish the work. Once again we thank you for giving us the opportunity to remedy the matter and hope that this will not spoil our good commercial relationship. Yours sincerely [votre nom] [votre fonction]