Section 3--119. Other Writings Affecting Instrument

(1) As between the obligor and his immediate obligee or any transferee
  the  terms  of  an  instrument  may be modified or affected by any other
  written agreement executed as a part of  the  same  transaction,  except
  that  a  holder  in  due course is not affected by any limitation of his
  rights arising out of the separate written agreement if he had no notice
  of the limitation when he took the instrument.
    (2) A separate agreement does  not  affect  the  negotiability  of  an
  instrument.