Ward just signed a new 1 year contract at around 1.3 million. Trading him for a minor leaguer effectively removes 1.3 million from the books. Ward would NOT have been a free agent after this season. He has another arbitration year ahead of him. The Astros needed to get rid of an outfielder because they currently have Berkman, Hidalgo, Hunter, Merced, Biggio and Hall (acquired as a Rule 5 draftee in the Cruz deal), not to mention Lane. If Hall is a total bust in spring training he'll be offered back to the D'backs, otherwise he'll stay on the roster unless the Astros work out a deal with Arizona - then he'll go down to the minors.
That's the plan, not sure what the 'Stros will have to give up ($$$ or low-level non-prospect) but look for something to be finalized during ST.