HR or upper management might help you, but are not required to: the company does not have to look into this and does not have to take action to correct the situation. It would be voluntary for them to addess the situation. You can certainly ask for help, but cannot guaranty you will get it.The only way to guaranty you get the money would be to sue for it and prove (e.g. by credible testimony of yourself and others; by time sheets and other documentation) that you were not paid what you should have been paid; if you prove this in court "by a preponderance of the evidence" (or that it is "more likely than not" that you are right), a court can order you payment. You could either sue the employer, for not paying you correctly; sue the manager, for essentially stealing from you; or sue both. This could obviously affect your relationship with your employer if you sue or implicate them, rather than suing only the manager who did this to you. If you sue the employer, you can likely sue in CA, since they have a presence there; but you'd most likely have to sue the manager in CO, where he did these things and (presumably) is located.