Can I be sued for this?

Back in August I used my boyfriend's checking account (without his authorization) to pay a credit card bill. The transaction didn't go through but it caused his account to overdraw as well as checks to bounce. I paid him back the amount that his account was overdrawn (over $800). I paid him the money back by depositing the money into his checking account. I no longer want to be with this guy, but every time I try to break up with him, he threatens to have me arrested and press charges against me for this and say that I never paid him the money back. Can he do this? Im not sure but isn't it illegal to threaten to press charges againgst someone? I feel like he is blackmailing me. He says he's gonna have his lawyer call my job and have the cops come arrest me at work. I have EVERY receipt from all the deposits I made into his account after this incident.I know this was a stupid mistake on my part but I also know that I paid him back EVERY penny. Please help!!!