Make Procedure To Check Balance &> Or &< Totalcost

 helo  all...,

i have create procedure can decrease totalcost from order table(database:games.dbo) with balance in bill table(database:bank.dbo). my 2 database in same server is name "boy"


i have 2 database like: bank.dbo and games.dbo 

 in games.dbo, have a table name is order(user_id,no_order,date,totalcost)

in bank.dbo, have a table name like is bill(no_bill,balance)

this is a list of bill table

no_bill            balance

111222            200$

222444            10$ 

this is a list of order table 

user_id            no_order            date            totalcost

    a                     1                  1/1/07             50$

when customer insert no_bill(111222) in page and click a button,  then bill table became

no_bill            balance

111222            150$

222444            10$

when customer insert no_bill(222444) in page and click a button, then message "sorry, your balance is not enough" 



mystore procedure like:
ALTER PROCEDURE [dbo].[pay]
(
    @no_bill AS INT,
    @no_order AS int,
    @totalcost AS money
)
AS
BEGIN
    BEGIN TRANSACTION
   
        DECLARE @balanc AS money

       
        SET @balanc= (SELECT [balance] FROM Bank.dbo.bill WHERE [no_bill] = @no_bill)

        UPDATE [bank.dbo.bill]
        SET
            [balance] = @balanc - @totalcost
        WHERE
            [no_bill] = @no_bill


    COMMIT TRANSACTION
END
 it can decrease money in bank, but i want it ceck money if balance > totalcost, so balance-totalcost,
if balance<totalcost,so error message"sorry, your balance not enough"
is it can make in procedure?
thx...