Look your boss dead in the eye, don’t call em. Do it in person to emphasize just how serious you are.
Look them dead ass in the eye and tell them, “You can’t afford to pay me overtime? Then I can’t afford to continue working here. Pick your poison. Be down two employees, or pay me correctly.”
Seriously. Your boss flat out lied to you. If they can afford two employees for that job, they can afford to pay you overtime after the other employee quits.
Also, if they fire you for this, take them to court. You will win.
Edit: This isn’t my question. A lot of you have left fine answers. Please make sure to tap the question to go directly to it, so you can leave an answer. All these answers in my comment section will never be alerted to the one who posted the question.