First make sure that the POSNR value is populated correctly in it_vbfa.
Then do the following (do minor modifications where necessary):
loop at it_sum into wa_sum.
read table it_vbfa into wa_vbfa
with key vbeln = wa_sum-vbeln
posnr = wa_sum-posnr.
if wa_vbfa-vbtyp_n = 'N'.
wa_sum-fkimg = wa_sum-fkimg * -1. " Make it negative
endif.
"Add the record to new internal table
read table it_final assigning <fs_final>
with key vbeln = wa_sum-vbeln.
if sy-subrc = 0.
" Already record exists
<fs_final>-billing_qty = <fs_final>-billing_qty + wa_sum-fkimg.
else.
wa_final-vbeln = wa_sum-vbeln.
wa_final-billing_qty = wa_sum-fkimg.
append wa_final to it_final.
endif.
endloop.