I'm 3 months into the interview process for a pretty prestigious research lab. It'd be a great gig. The hiring manager has told me that my peer references were great and I'm by far the most qualified applicant, but their HR requires that they speak to my current manager and get a qualitative reference about my work, not just to verify employment dates.
However, my current manager is pretty abusive, which is why I'm leaving. He's told me and others in my lab that he'll never give a good reference and he'll fire anyone who he finds out is looking for another job.
I can't afford to be fired for reasons I can't get into here. How should I go about withdrawing my application, but letting them know that it's just because of this reason, and I'd love to work there if they could change this rule?