Try this
DECLARE @A TABLE
(
ARTICLE_ID VARCHAR(10),
COST NUMERIC(10, 2),
qty_should_be INT
)
DECLARE @B TABLE
(
ARTICLE_ID VARCHAR(10),
COST NUMERIC(10, 2),
qty_present INT
)
INSERT INTO @A(ARTICLE_ID, COST, qty_should_be)
SELECT 'ART0000024', 180, 100
UNION ALL
SELECT 'ART0000024', 1079, 1000
INSERT INTO @B(ARTICLE_ID, COST, qty_present)
SELECT 'ART0000024', 144, 100
UNION ALL
SELECT 'ART0000024', 1099, 1000
SELECT A.Article_ID, Sum(qty_should_be) - Sum(Qty_Present)
FROM @A A
LEFT OUTER JOIN @B B ON A.Article_ID = B.Article_ID
group by A.Article_ID