You may have to repeat the calculation in the where clause since it isn't really a column....
WHERE (wbprodSTOCK - (SELECT Coalesce(SUM(DetailQuantity),0) FROM WBorders INNER JOIN WBorderdetails ON WBorderdetails.DetailOrderID = WBorders.OrderID WHERE OrderDate > wbprodSTOCKDATE AND DetailProductID = wbprodID)) > 0
Another option is to use HAVING instead of WHERE, which allows you to filter based on columns you have created in the SQL statement, but that can be pretty inefficient if you have a very large database.