All they had to do was tender him. They could have gotten him for $3 million but even that was too much for a guy who might not pitch this season (or only for two months). And you don't really want to offer him a second guaranteed year before you see him pitch again.
He won't even make more if he comes back because the Cubs (or whoever trades for him) will still have one year left of arbitration, he'll get a little service time bump but probably can't have enough of a 2020 season for more than that.
I didn't like the move at the time, I like it okay now but only because of the unlikable financial path the team has chosen.