You didn't provide enough detail regarding
datatype
of your table
attributes and
variable, let me guess you are using
datatype money
for your
[Money] field. If i am right, try using the following code:
UPDATE CustomerBankInfo
SET [Money] = ([Money] - CONVERT(money,@orderValue)
WHERE CustomerBankInfo.CustomerID = (SELECT CustomerID
FROM Customer
WHERE FirstName = @custFName
AND LastName =@custLName)
AND ([Money] > CONVERT(money,0))