Don't have SQL Server 2000 to test with, but you should be able to get it done using a plain subquery, something like;
SELECT SUM(TotalSales) TotalSales, SUM(TSalesAftComm) TSalesAftComm,
SUM(NumOfSales) NumOfSales, SaleDate
FROM (
SELECT
Emp_ID,
sum(SaleA+SaleB) as TotalSales,
sum(SaleA+SaleB-CommA-CommB) as TSalesAftComm,
count(Emp_ID) as NumOfSales,
SaleDate
FROM Sales (nolock)
WHERE SaleDate>='2014-03-15 00:00:00'
GROUP BY SaleDate, Emp_ID
HAVING
sum(SaleA+SaleB) > 10000
) z
GROUP BY SaleDate
ORDER BY SaleDate