Dowd clearly lied. Did he just put himself in legal jeopardy?
Had he lied as part of an official proceeding, (i.e. misrepresented something to a judge or lied under oath) he would be in deep ----. Unfortunately i don't think there's anything special about being a lawyer that prevents you from lying to the media.
Now i've seen speculation that because the tweet contained evidence relating to obstruction, that Dowd can now be put under oath by Mueller and asked who wrote the tweet. If he then admits to not having written the tweet, then his statement that he did write it would also be obstruction. I would like to see this theory tested.
At the end of the day, there is very little reason to believe that Dowd wrote the tweet. Of all the tweets Trump has written, the one that happens to place him in legal jeopardy just happens to be the one his lawyer wrote? It doesn't make sense; no criminal defense attorney wouls be this stupid. The tweet is obviously in Trump's voice and if it ever matters i'm sure it can be figured out.