Make Procedure For Stock-quantity

 helo alll...,
this is my data:

my table is item(productid,stock) ,order(customerid,no_order), and orderdetail (no_order,productid,quantity)
 example:
 order and orderdetail displayed in gridview....

order is displayed like this:
                 customerid            no_order
detail                  A                        1
detail                  B                        2 
 when i click detail in row 2, it's display orderdetail:

no_order               productid              quantity    

  2                            c1                       2

  2                            p1                       3

 i have make all this is ok. but i want to decrease stok in item with quantity in orderdetail.
my code in procedure like:CREATE PROCEDURE [dbo].[order_item]
(

@productid AS varchar,
@quantity AS INT
)
AS
BEGIN
BEGIN TRANSACTION
DECLARE @no_order AS INT
DECLARE @stock AS INT

INSERT INTO [Orderdetail]
(

[ProductId],
[Quantity]
)
VALUES
(

@productId,
@quantity
)

SET @no_order = SCOPE_IDENTITY()
SET @Stock = (SELECT [Stock] FROM [item] WHERE [ProductId] = @productId)

UPDATE [item]
SET
[Stock] = @Stock - @quantity
WHERE
[ProductId] = @productId
COMMIT TRANSACTION
END
return
 it can't work..

how about his true code in store procedure?
ok..., thx..