Heyward will easily get more than 20M per season. Let some other team pay for defensive WAR that isn't close to being a perfected measure.
Here is Bill James on Wins Above Replacement, perhaps the hottest advanced statistic in the game right now:
"Well, my math skills are limited and my data-processing skills are essentially nonexistent. The younger guys are way, way beyond me in those areas. I’m fine with that, and I don’t struggle against it, and I hope that I don’t deny them credit for what they can do that I can’t.
"But because that is true, I ASSUMED that these were complex, nuanced, sophisticated systems. I never really looked; I just assumed that the details were out of my depth. But sometime in the last year I was doing some research that relied on these WAR systems, so I took a look at them, and ... they’re not very impressive. They’re not well thought through; they haven’t made a convincing effort to address many of the inherent difficulties that the undertaking presents. They tend to get so far into the data, throw up their arms and make a wild guess. I don’t know if I’m going to get the time to do better of it, or if it will be left to others, but ... we’re not at anything like an end point here. I assumed that these systems were a lot better than they actually are."
So basically, even Bill James says to not take WAR overly seriously. Heyward is not worth his WAR. I'm fine with another team paying him 25 per year. Now if he wanted to stay at Freemans deal then by all means we should have kept him.