The offered wage must be in the LMO application, and would have to be presented along with the contract/letter of employment, as part of the documents submitted for the TWP. Any disparity would be noted and cause for rejection.
As well, your employer would be foolish to give you a signed contract at one wage and then not actually pay that wage to you. Beyond the fact that your employer would open themselves up to legal problems, your employer could be audited any time, and any discrepancy would be cause for revoking the LMO/TWP, as well as making them ineligible to hire foreign workers in the future.
There is a lot of political hot air right now about "protecting" foreign workers from abusive employers, and underpayment is one of the hot issues. Don't know if it will really translate into action, but it's not something I would want to gamble on...
Good luck - hope you can negotiate a raise out of this
