If you have an actual written employment contract for a definite time (e.g. a one-year contract) signed by both you and your employer, you can enforce its terms, and the employer would have to pay you any wage in the contract. If not, they may change your wage at will, reduce it in whole or part, pay you different wages for different shifts or work, etc., subject only to having to always pay minimum wage and having to pay overtime (time-and-half) for any work past 40 hours in a week. (If you are paid two different wages, overtime is based on a blended rate: if you earch $30/hour for 20 hours and $10/hour for another 20, then your blended rate is $20/hour so overtime would be calcualted off that.)