You also don't have any sales for acct's 5/6 in June.

How does this work for you?

SELECT c.ACCT_NO, c.L_NAME, sum(IsNull(p.SALE_PRICE,0))

FROM Creditor c

LEFT JOIN Purchases p

ON p.ACCT_NO = c.ACCT_NO

AND p.DATE >= @fromDate

AND p.DATE <= @toDate

GROUP BY c.ACCT_NO, c.L_NAME

HAVING sum(IsNull(p.SALE_PRICE,0)) <= @minPurchase

ORDER BY c.L_NAME