Typically an employer cannot unilaterally alter the terms of an employment contract, according to the American Bar Association. Under basic contract law, all parties must have a meeting of the minds in order to amend a contract.
Under basic contract law principles, a contract between two parties cannot be unilaterally altered by one party, which in this case is the employer, according to the American Bar Association.
It is important to note however that most employees are not governed by an employment contract but are considered to be at-will employees, according to the Equal Employment Opportunity Commission. In most states, an employer can alter the terms of employment or terminate an at-will employee at any time without cause. However, U.S. federal law does prevent an employer from discriminating against an employee based on race, ethnicity, country of origin, religion, sex, age or disability.