SELECT COUNT(DISTINCT d.PONum) AS POCount,
       ISNULL(SUM(d.DocExtCost), 0) AS POValue,
       CASE v.Buyer_c
            WHEN 'Person5' THEN 'Person4'
            ELSE v.Buyer_c
       END AS BuyerID

FROM [Dbo].PODetail AS d

INNER JOIN [Dbo].Vendor AS v
        ON (d.Company = v.Company AND
            d.VendorNum = v.VendorNum)

WHERE v.Buyer_c IN ('Person1', 'Person2', 'Person3', 'Person4', 'Person5')

GROUP BY CASE v.Buyer_c
            WHEN 'Person5' THEN 'Person4'
            ELSE v.Buyer_c
         END

ORDER BY v.Buyer_c   <-- This order by doesn't work

Column "Dbo.Vendor.Buyer_c" is invalid in the ORDER BY clause because it   is not contained in either an aggregate function or the GROUP BY clause.*

GROUP BY CASE v.Buyer_c
        WHEN 'Person5' THEN 'Person4'
        ELSE v.Buyer_c
     END,
     v.Buyer_c