Where there is an employment contract, the rights and obligations of the employer and the employee are governed by the terms of the contract. No one here has reviewed the contract, so no one here can tell you how its terms govern in the situation that you are concerned about. You will need to take your contract to a local employment attorney and have a consultation about the nature and extent of your contractual protections.
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If you have a contract which spells out the raise for a specific period of time, you may have a claim for the continued raise. However, if you are an at will employee, your employer can change your salary at any time. You would need to see an attorney to see if there are contractual rights in existence.