updating column in table 1 by calculating using columns in table 2

Hi,

How can I do the following please?

I want to update a total column in 1 table using 2 columns in another table for multiple rows.

This is the rough query, except I get the error
'Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.'

eg

update tbPay
set total = (
SELECT      SUM(mPrice * iQty - ( (mPrice * iQty) * CONVERT(float, vCustom1)/100)) as CostIncDiscount
FROM         dbo.tbItem INNER JOIN
                      dbo.tbPurchase INNER JOIN
                      dbo.tbWholesalerP ON dbo.tbPurchase.iPurchaseId = dbo.tbWholesalerP.iPurchaseId INNER JOIN
                      dbo.tbPay ON dbo.tbPurchase.iPurchaseId = dbo.tbPay.iPurchaseId ON dbo.dbo.tbItem.iPurchaseId = dbo.tbPurchase.iPurchaseId
WHERE  (dbo.dbo.tbItem.iProductId BETWEEN 1 AND 4) AND
                      (dbo.dbo.tbItem.dtCancel IS NULL) AND   dbo.tbWholesalerP.dtDispatch > '01 dec 2012'
GROUP BY dbo.dbo.tbItem.iPurchaseId)
FROM         dbo.tbItem INNER JOIN
                      dbo.tbPurchase INNER JOIN
                      dbo.tbWholesalerP ON dbo.tbPurchase.iPurchaseId = dbo.tbWholesalerP.iPurchaseId INNER JOIN
                      dbo.tbPay ON dbo.tbPurchase.iPurchaseId = dbo.tbPay.iPurchaseId ON dbo.dbo.tbItem.iPurchaseId = dbo.tbPurchase.iPurchaseId
WHERE (dbo.dbo.tbItem.iProductId BETWEEN 1 AND 4) AND
                      (dbo.dbo.tbItem.dtCancel IS NULL) AND   dbo.tbWholesalerP.dtDispatch > '01 dec 2012'