Hi Volker,
Please check user exit USEREXIT_MOVE_FIELD_TO_COBL in Program MV45AFZB.
This is processed for dialog and bapi sales order creation/update.
Add this coding:
if ch_cobl-kostl isinitialandnot us_vbapkom-kostl isinitial.
ch_cobl-kostl = us_vbapkom-kostl.
endif.
There might be other checks you have to do to distinguish between online and BAPI processing.
/Michael