Found work around by myself in MV45AFZZ.
USEREXIT_FIELD_MODIFICATION
STATICS lss_vbpa TYPE vbpavb.
DATA ls_xvbpa TYPE vbpavb.
IF sy-dynnr = '4701'.
IF screen-name = 'KUAGV-KUNNR'.
CLEAR ls_xvbpa.
READ TABLE xvbpa INTO ls_xvbpa WITH KEY parvw = 'SP'.
IF ls_xvbpa IS NOT INITIAL.
IF ls_xvbpa <> lss_vbpa.
PERFORM preisfindung_gesamt USING 'C'.
lss_vbpa = ls_xvbpa.
ENDIF.
ELSE.
IF lss_vbpa IS NOT INITIAL.
PERFORM preisfindung_gesamt USING 'C'.
CLEAR lss_vbpa.
ENDIF.
ENDIF.
ENDIF.
ENDIF.: