As
@NotTheRocketman stated chances that we sign him to an extension have increased quite a bit, but let's entertain the hypothetical case that they can't agree on a extension and Army decides to move him:
The ask at last trade deadline was reported to be a return with 2 first round picks value. So for example either an actual first round pick+a good prospect, a 2nd + elite prospect or a really good young rosterplayer (preferably a future top-4 defenseman with top pairing upside - see the Mittlestadt/Byram trade), etc. I don't know if the ask was realistic or if the ask has lowered as he has less term now, but can be moved with an extension.
I guess a 1st (24 1st probably prefered, but the 25 1st is possible if Yzerman doesn't want to budge) + Wallinder for Buch extended and/or retained is probably enough at the next deadline. The asked return for a trade during the preseason/draft is probably a bit higher. If Buch tells Army he doesn't want to extend (again IF), a 1st, Wallinder + Berggren/pick/prospect would probably be enough for Buch extended/retained.