Updating quantity field BSEG-MENGE in transaction vf01

Hi,

We are posting debit memo to customers. The requirement is BSEG_MENGE field should get updated with the billed quantity for a specific account. So we are exploring to use user exit EXIT_SAPLV60B_004(SDVFX004). In this exit in debugger we tried to manually change values for the field xaccit-menge and xaccit-xmfrw. Though the values are entered, they are not updated in the posted accounting document. When we tried to change other fields like xaccit-ebeln or xaccit-xref1, they are getting updated anyabody who can help on this is most welcome. WE are currently on ECC 6.0 version.

With regards,

Suresh E