Hi Silent Knight,
Try this one:
SELECT SUM(ISNULL(T0.DocTotal,0)) AS OpenTotal
FROM (select * from ODRF WHERE ObjType='17' AND DocStatus <> 'C') T0
WHERE T0.CardCode=$[$4.0.0]
Just make sure you have set Customer Code already in the Sales Order before executing the query because of $[$4.0.0]
Thanks!