When creating an ASN document for a Schedule Agreement Release (SAR) in SUS the system will propose the full quantity for all items, even when for some of the items ASN has been created for the full quantity. To consider the already ASN-ed quantity you can implement user-exit MEETA001 in ECC with this sample coding:
data: ls_ekeh like i_ekeh,
lv_menge like i_ekeh-menge.
loop at i_ekeh into ls_ekeh.
check not ls_ekeh-updkz is initial.
lv_menge = ls_ekeh-menge - ls_ekeh-dabmg.
if lv_menge eq 0.
clear ls_ekeh-updkz.
else.
ls_ekeh-menge = lv_menge.
endif.
modify i_ekeh from ls_ekeh.
endloop.
You can adjust the coding to your requirements when needed. As an alternative you can implement the BADI BBP_SUS_FILTER_SCHAR on SUS side to achieve the same effect.