-- 이런 류의 질문엔 반드시 정렬기준이 있어야 합니다.
SELECT a.site_emp
     , NULLIF(a.invoice_amount
       , LAG(a.invoice_amount) OVER(PARTITION BY a.invoice_id ORDER BY b.정렬기준항목)
       ) invoice_amount
     , b.amount
  FROM invoice a
     , check b
 WHERE a.invoice_id = b.invoice_id
;