What are grounds for defamation and slander from my boss?

I was just let go from my job in Michigan. Before i took the job i recieved an anonymous text, leter finding out it was the boss' soon to be ex girlfriend, stating he does not like to pay his employees and he abuses women. I was hired at $12 an hour. I believe the reason i was let go is because they worked out their relationship and he no longer needed me. I was told i didn't work hard enough, which is not true and i was even told i was doing great. I told him the reason i believe i was being fired and he came back threatening to pay me less than what we had agreed on if i continue the conversation. Also accused me of doing cocaine on the work place property. I do NOT use cocaine and i feel this can harm my reputation. I have everything in text too