Hi,
yes I found that this is standard behavior programmed, i.e. AAG is picked from customer master while passing to billing document. Only way I think is to use user exit
USEREXIT_FILL_VBRK_VBRP in program SAPLV60A
Below is sample code:
DATA: lo_order TYPE REF TO /dbm/cl_order.
FIELD-SYMBOLS: <ls_splhdr> TYPE /DBM/SPLIT_COM.
if xkomfkgn-/dbm/vbeln IS NOT INITIAL.
and xkomfkgn-/dbm/splnr IS NOT INITIAL.
PERFORM get_order_object_for_billing IN PROGRAM /dbm/saplorder_int
USING fkomfkgn-/dbm/vbeln
CHANGING lo_order.
CHECK lo_order IS BOUND.
* transport charg from order item to the invoice
READ TABLE lo_order->MT_SPLHDR_COM WITH KEY vbeln = fkomfkgn-/dbm/vbeln
SPLNR = fkomfkgn-/dbm/splnr
ASSIGNING <ls_splhdr>.
VBRK-ktgrd = <ls_splhdr>-ktgrd.
endif.