Try this one...
SELECT T1.ProductDescription,
Sum(Iif(T2.TransType='I',T2.Quantity,-T2.Quantity)) AS StockQty
FROM tblProduct AS T1
INNER JOIN tblTransaction AS T2 ON T1.ProductID = T2.ProductID
GROUP BY T1.ProductDescription
HAVING Sum(Iif(T2.TransType='I',T2.Quantity,-T2.Quantity))>0;