Invoice list accounting reference number Incorrect

Hi all,

      Im new to SD. When create invoice list for a Billing Document, the Reference Field of the Accounting Document(BKPF-XBLNR) is overwrite with invoice list no. My requirement when Invoice List is saved the system do not overwrite the reference number.

EX:

           VBRK                                                                   BKPF

Billing Doc     Reference                                      ACC.No          Reference

90287005      INV12150004                                         631          INV12150004

90287006      INV12150005                                         633          INV12150005

After Invoice List   creation                                             

                                                                            ACC.No           Reference

90287007       BN12150002                                            631      90287007

                                                                                        633     90287007

Any help is highly appreciated !

Thanks !