Remember that they need to get more than the equivalent of a first-rounder and a sandwich pick to make any deal worthwhile.
That's best case.
If the Astros don't offer him arb, as they didn't with Huff last year, they'll get nada.
If the Astros do offer arb and he signs with another team, any other free agents that sign with that team could affect the compenstion the Astros get. This is why the Padres got the Astros 2nd round pick rather than 1st in the recent draft. The Astros got Lee and Woody. Because Lee was the higher rated the Rangers got the Astros first round pick and the Pads got the Astros second round pick. Both teams did get supplemental first rounders as well. And, to make it messier, if the team signing Jennings is one of the worst 15 then draft pick compensation STARTS in the 2nd round though the Astros would still get a supp 1st rounder as well.