Nothing the Bills will do can make him stay. Sure, if you said we'll give you 25 million a year then he may consider it, but the other owners in the NFL are unlikely to just let such an escalation in salary occur without... interference even if the Bills would do it. You can't even fire McDermott (not that I would do that) to give him the HC job here because that would be such a dysfunction.
He's going to take a HC job. I think he's done an awesome job, but I don't think it's going to bottom out without him. There are other good OCs out there and working with Allen is going to be attractive (if they don't promote someone on staff)