Write your code in MV45AFZZ USEREXIT_SAVE_DOCUMENT_PREPARE.
check for xvbap-taxm1 and throw error message.
write the below code after the error message.
PERFORM FOLGE_GLEICHSETZEN(SAPLV00F).
CH_FCODE = L_C_FCODE.
SET SCREEN SYST-DYNNR.
LEAVE SCREEN.
other way is to ask your functional to add the field to in-completion procedure and then trigger this incompletion via exit, use the same save document prepare