Only [75 reputation](http://stackoverflow.com/privileges/set-bounties) is required to set bounties, and as soon as you offer the bounty, the points you outlaid are *immediately* removed from your reputation score.

So it's not hard to imagine that he had 75 reputation at one point (barely enough to set a bounty), and then set a 50 point bounty, which brought him back down to 25 reputation.

That's not *exactly* what happened here, because you said he only has 22 reputation at the time you posted the question, which is obviously a few points off from the required minimum of 25 that he should have. But that's easily explained through a downvote or two, or through the deletion of one of his posts that had received an upvote, *after* the bounty amount had been sliced off from his account reputation.

**Edit:** As further evidence that reputation can fluctuate rapidly (a consequence of [the recent changes](http://meta.stackexchange.com/questions/123319/recent-reputation-history-changes)), the user now has 27 reputation. And that will probably change some more as the rest of the Meta crowd downvotes his question.