They save a tremendous amount of 2018 money. Next year there's basically a $9m cap charge for the remainder of his dead money and that's it. Were he still on the roster next season the total cap cost was to have been $25.2m.
How you spend that is a little tricky - I'd first point out that you can't extend Dak, Zeke or any of the 2016 class until after the 2018 season is over under current rules. And even the Cowboys do plan ahead somewhat - there will be monster money for Dak waiting for him.
But before we can really talk about any headline free agents next year you'd need to know what's happening with higher-salary guys like Witten, Crawford, Dez, Sean Lee at that point, plus Zach Martin's upcoming monster deal, plus any extensions for free agents and 2015 class guys like Byron Jones. The
outgoing 2018 free agent list doesn't include any real core guys right now but a number who could emerge as players.
A simpler way to answer would be to say yes, they can definitely sign a premiere FA if they wanted to, or more than one, and make it work financially. That just hasn't been their thing lately, and IMO for good reason.
Example: Jason Pierre Paul was a guy I thought they might target for a high but manageable price, and he ended up with a deal that averages about $15m a season ($30m was guaranteed on a 4 years). That's real money not headline fluff. Great player but this is a guy who's missed 12 games in the last two seasons.
Bottom line is that free agency for impact players is pretty awful with this new CBA, it's just far easier for teams to retain top talent. Players can't really hold out effectively anymore, there's now the 5th-year option for Rd 1 guys, things like that...I think the Cowboys' recent FA strategy is the right adjustment, they just have to hit on a couple more defensive picks instead of whiffing on the DLaws, Gregorys, Claibornes or screwing around with FA clowns like Greg Hardy and Rolando McClain.