Main question: When, if ever, is it appropriate for a potential employer to ask you to contact another employer (whose offer you've already accepted) to tell them you're reconsidering prior to extending a formal offer?
Particular situation: A little while back I accepted an offer to work for Company XYZ. Due to various logistical reasons, my start date is not for another few months. Recently, I was independently and unexpectedly approached by someone (let's call him Bob) with a new business proposition for me where I would join his small company in the same field as XYZ. I would be one of the first few employees of this new venture.
Bob is a former principal at XYZ whom I originally met while doing my own due diligence prior to accepting the offer with XYZ. He hasn't been involved with XYZ for many years, but due to how we met, he knew prior to approaching me that I had a relationship with them and had accepted an offer there, and I raised this issue explicitly with him in our conversations as well.
Bob's proposal was interesting, we get along very well, and it would seem to provide me an opportunity to grow professionally in a way that I wouldn't as readily be able to at XYZ. We informally agreed on rough compensation and he sent me an informal email outlining this. I then asked him to provide me with a formal offer for review as a next step.
At this point, Bob became a bit cagey and he asked me to first call up XYZ and let them know I was reconsidering my options prior to him formally extending me an offer. His stated reason is that this is for "politeness" and he wouldn't feel comfortable otherwise. I told him I wasn't immediately comfortable with this tack and would have to think it over.
Is this an acceptable approach on Bob's part to this situation? Am I doing something incorrect or ill-advised?
It seems obvious that doing as Bob suggests would immediately reduce my leverage and also create the situation where I could end up without either opportunity to consider. I'm also concerned about what it might signal about Bob's future behavior.
I didn't go looking for the opportunity with Bob and I would be able to give XYZ several months of advance notice if I did choose to withdraw, so while I would really regret having to go back on my word, I don't feel I've done anything untoward here.
What's the best way to proceed?