What would be my best bet for this situation?

Damn. Okay that's a predicament. I worked at an electronics store as well. The source electronics. one of the employees spread lies about me as well, when I started applying he got the job of ASM so I knew my time was severely limited. I was called in for something I didn't do that he blamed me for and I also had an interview pending. I went in and took the blame for what he accused, not able to defend myself with ample evidence and left. I was told I broke regulations and would soon have to leave if it continued. So while my interview was pending I passed in an alternate reference. That reference being the ASM. Bold move, I don't advise trying this yourself but I read into people. He was hungry for attention but he can't speak straight when in control, so he said good things about me. That's just my story. But I advise you to take precautions and approach the manager calmly. Tell him what's been going on, and that you'll leave if the ASM continues to bastardise your reputation.