FACTURE_ENABLE_NEGATIVE_LINES = '1'

  valeur brute (10, -10, 55)   -> lignes stockees [-10.00, -10.00, -55.00]  total -75.00
  valeur negee (-10, 10, -55)  -> lignes stockees [-10.00, -10.00, -55.00]  total -75.00