Handling collaboration requests from the authors of double-blind reviewed papers (after submitting the review) (I apologize if this is a duplicate, but I was not able to find any similar case on AcademiaSE)
I am finding myself in a very odd and unpleasant situation. The subject is &quot;ethical&quot; peer review in Computer Science.
I am a sub-reviewer of a conference. As such, I was assigned some papers to review (in double-blind format); however, for the papers I am assigned, I am able to see who the other reviewers are, and also their reviews on these papers. As a sub-reviewer, my name does not appear on the listed TPC of the conference.
Among the papers I am assigned, there is Paper A. I reviewed this paper and I liked it, albeit it had some flaws. After submitting my review, I was able to read the other reviewers' remarks - some were positive, some negative. My (positive) comments allowed the paper to pass the first round of review.
Now the problem: I received an email from a Researcher X who I did not know, asking a collaboration proposal; the email had Paper A in the attachment. Apparently, Researcher X found it relevant to include a proof that they're working on something to corroborate the collaboration request. I would be very willing to collaborate with X, as I consider their research group to be strong.
You can easily understand my awkward situation: what to do?
Things to take in mind:

My review has already been submitted and evaluated. Hence, I cannot turn down the review-request.
I can turn down the collaboration request, but I would lose a significant opportunity for my academic career.
Even if I warn the TPC of the conference to &quot;retract&quot; my review, I was still able to see the reviews of the other reviewers (as well as their names).
Moreover, my review has already had an impact on the future of the paper.
My name did not appear on the TPC list, meaning that Researcher X acted in good faith.
Given the timeline there is no ethical concern. You don't need to retract your review and you can agree to collaborate.
However, you should decline to review paper A if it goes to a next round. And in the short term you should refrain from telling the authors of your review. I don't see an issue in telling them later, after a final decision is made by others.
You might want to inform the program chair of the situation, giving the timeline. I doubt that they would have any complaint nor any reason to act.
Given what you say, the events seem to be entirely independent. Serendipity happens.