Przerobione na kolanie, sprawdź
SELECT
t.tw_symbol as Symbol,
t.tw_nazwa as Nazwa,
SUM(a.mr_ilosc - Isnull(r.mr_ilosc, 0))AS 'Ilość dzień',
st_stan as 'Stan obecny'
FROM
dok_magruch a
INNER JOIN tw__towar t ON a.mr_towid = t.tw_id
INNER JOIN tw_Stan ON t.tw_Id = tw_Stan.st_TowId
LEFT JOIN ((SELECT Isnull(SUM(c.mr_ilosc), 0) mr_ilosc, c.mr_doid FROM dok_magruch c WHERE c.mr_data <={D: Na dzień} GROUP BY c.mr_doid)) r ON r.mr_doid = a.mr_id
WHERE
a.mr_data <={D: Na dzień}
AND tw_Stan.st_MagId = 1
/*AND a.mr_ilosc > Isnull(r.mr_ilosc, 0)*/
AND a.mr_magid = 1
GROUP BY
t.tw_id,
t.tw_symbol,
t.tw_nazwa,
st_stan
having isnull(SUM(a.mr_ilosc - Isnull(r.mr_ilosc, 0)),0)=0
ORDER BY
tw_id