This likely will depend on what your agreement is with your employer. Do you have a written contrat for employment? The problem you might face is that your employment may be "at will" which essentially means they can terminate you for any reason (with exception of protected categories), let alone offer you less hours. If you have a contract or agreement that requires your employer to pay you for 40 hours each week, then you might have an issue. Department of Labor is often a good resource for folks in your situation.