Professor to whom I broke my promise is now a collaborator. What to say? 7 years ago, I got a PhD offer from professor X. Thinking that I would have no chance with other applications, I accepted it. I signed the offer, and the HR was helping me with the enrollment process.

But then I unexpectedly got another offer, and I broke my promise. I apologized in the most sincere way my poor English could express. X briefly said he understood. This is morally wrong, but not what I want to ask.

One of my collaborator just joined the department of X. He wants to involve X in our on going work. I have no problem with that, X is nice, and he can make a good contribution. But what should I say to X to break the ice?


Should I start by apologizing again?
Should I just pretend that I don't remember about him? This is good if he already forget about me. But if he still remembers about me because I caused him so much trouble, then I will appear as a horrible person to him.


What should I do?
I understand that this situation is somewhat awkward, but I think it is important not to overthink this. It was, after all, a fairly long time okay, and at the time the professor said he understands, so you may be taking this incident harder than he is.


  But what should I say to X to break the ice?


I probably wouldn't mention anything, at least not at the beginning and definitely not over e-mail. Bring it up at an opportune time, or not at all.


  Should I start by apologizing again?


No. See above. Even if he remembers you (which is not at all guaranteed), starting a collaboration with "remember how I reneged from our agreement last time?" seems supremely awkward.
Should the collaboration take off and you meet more often and personally, it might be nice to apologize again, optimally in a social setting. Alternatively, if he brings the topic up, you can and should apologize. However, don't make a big deal out of it if he doesn't.


  Should I just pretend that I don't remember about him? This is good if he already forget about me. But if he still remembers about me because I caused him so much trouble, then I will appear as a horrible person to him.


You don't need to pretend you don't remember him. Just interact with him normally - even if you don't start a conversation about your last interaction immediately  (and he remembers you), he will probably not assume that you forgot about him, but that you are trying to not make the situation awkward. He won't consider you a horrible person for this.