I was hired as sales manager recently. after 2 months they are demoting me to supervisor. is that lawful?

consistently the invitation was a supervisor. which i repeatedly not accept the offer. when ask what can change my decision and said is the sales manager. then they gave in. after 2 months they said that i did not qualify and wants me being demonted after resigning for my job. i felt trick. what can i do? is that lawful?