Can my employer legally not pay me overtime instead telling me to leave early over the next few weeks? 

Employers may decide how many hours that they want their employees to work.  The employees have a choice to either work those hours or quit as they are employees at will and may leave at any time that they want. In most states, there are wage statutes that state that employers are required to pay certain amounts to employees if they work overtime hours and that their failure to pay employees for work will result in damages  (i.e. some state give the employees double damages).  If you worked overtime and have not been paid for the hours that you have worked, then i suggest that you hire a lawyer to file a claim against the employer.