A pilots job is to say no when things aren't right. It's an easy job when everything goes smoothly, but the money is earned when things are tough. Your boss sounds unreasonable. Don't let his reaction influence future decision making. Whether 80#s over MATOW is worth canceling the flight or not is irrelevant. You are the PIC and you made a decision based on the safety of the flight. You made a decision that was right for you, and you should be proud of the fact that you stood your ground. Nice job.