DATA: lv_dmbtr_s TYPE dmbtr.
*      lv_dmbtr_h TYPE dmbtr.
SELECT SUM( CASE WHEN shkzg = 'S' THEN dmbtr
                                  ELSE - dmbtr END ) AS dmbtr
  INTO @lv_dmbtr_s
  FROM bsid
  WHERE kunnr EQ @likp-kunag.