It's no more of a conflict of interest than when a state court judge goes into private practice with a law firm. Or when a prosecutor goes into private practice as a criminal defense lawyer. It happens all the time. Why would anyone ever take a public job if they knew they could never take another job in a related field after that?
And, you are correct, the job came available first and then he announced he was leaving Congresss (is that really "retiring"? Has he used that word?). My understanding is that the firm came to him, he announced that he was going to interview with them (as per the new lobbying rules), and then they came to a deal - which led to this second announcement.
Believe me, I am sorry to see him go, but, when you've been a Congressman for over 20 years and and you're still only making slightly more than a first year associate at a big DC law firm, I can't begrudge the guy taking a better paying job.