Need Some Logic Help

Can anyone see where my logic may have gone a stray?

A user would be attempting to update a gridview row, I'm storing the amount of an item and the qty they purchased. If the order was for $150 and the grid has two item 1 x $50 and 2 x $50 the order already totals $150, so if the user tries to change the first item to a qty of 2 x $50 the total would be $200 and this would be more than the total payment or $150.

I want to then tell them they cannot do this.

@AP_ID Int,
@AI_ID int,
@PurchaseAmount money,
@PurchaseQTY int,
@LastUpdate datetime,
@LastUpdateBy nvarchar(50),
@Receipt_ID int

AS

declare @current_item_total decimal(18,2)
declare @new_item_total decimal(18,2)
declare @total_payments decimal(18,2)
declare @current_purchase table
(
receipt_id int,
purchase_amt money
)
INSERT INTO @current_purchase SELECT  tblPurchase.Receipt_ID,(tblPurchase.PurchaseAmount * tblPurchase.PurchaseQty) as purchase_amt
FROM         tblPurchase INNER JOIN
                      tblReceipts ON tblPurchase.Receipt_ID = tblReceipts.Receipt_ID
WHERE tblReceipts.Receipt_ID=@receipt_id

--Get total already saved to record
select @current_item_total = sum(purchase_amt) from @current_purchase

--Get total amount of payment saved to record
select @total_payments = sum(tblReceipts.AmountPaid) from tblReceipts where tblReceipts.receipt_id=@receipt_id

--Get total amount user is trying to save to record
set @new_item_total = (@PurchaseAmount * @PurchaseQTY)

--If current total plus new total is greater than total payment, tell user they cannot do this.
IF ((@current_item_total + @new_item_total) > @total_payments)
BEGIN
SELECT 'You are attempting to add more than your total payment.' AS MESSAGE
END
ELSE
BEGIN
--UPDATE ROW
UPDATE tblPurchase SET [AI_ID]=@AI_ID,[PurchaseAmount]=@PurchaseAmount,PurchaseQTY=@PurchaseQTY,LastUpdate=@LastUpdate,LastUpdateBy=@LastUpdateBy WHERE [AP_ID]=@AP_ID
SELECT 'Item was updated.' AS MESSAGE
END