Section 5-1103. Written agreement for modification or discharge

An
  agreement, promise or undertaking to change or modify, or  to  discharge
  in whole or in part, any contract, obligation, or lease, or any mortgage
  or  other  security  interest in personal or real property, shall not be
  invalid because of the  absence  of  consideration,  provided  that  the
  agreement,  promise  or  undertaking changing, modifying, or discharging
  such contract, obligation, lease, mortgage or security  interest,  shall
  be  in  writing  and  signed  by  the party against whom it is sought to
  enforce the change, modification or discharge, or by his agent.