Not a bona fide answer, in that I am not confident enough to provide you with a reliable example of what you should be saying, but I can definitely tell you how you should not be saying it (despite some suggestions in the comments to your question):
- Any sentence that starts by a word expressing disagreement.
- Anything that hints at an actual error made by your boss: "間違っている", "間違い" or, Amaterasu forbid: "違う！" etc.
In fact, the two points above are probably good guidelines for any argumentative discussion with a Japanese person.
I know the whole resolution by consensus thing is a bit of a tired cliché about Japanese society, but there is a reason it became one: you can safely expect Japanese in general, your business associates in particular, and your boss most definitely, to dislike direct confrontation even (particularly) when they are factually wrong.
The standard recommended way to handle both a personal or a business disagreement (there is no such thing as a factual error in Japanese, only differing viewpoints ;-) is:
- Start by agreeing wholeheartedly (そうですね etc).
- Introduce your correction/viewpoint, as if it was some last minute detail you just thought of, that was of no importance whatsoever, or as an improvement, rather than a correction.
Of course, I am drawing very broad strokes here and standard use-your-better-judgement disclaimer applies, but I think you can't really go wrong with this approach.