It's $5m v. $8m. Pay the buyout, pay his salary. There's a lot in it for Moncada if he can get on the field by around the 18th of July, or whatnot. Mutually beneficial. If he could OPS .750 in a playoff run, and play good 3B defense, he might be one of the higher FA guys. Even if he doesn't, then he plays 6 weeks with the White Sox, takes up a roster spot from a prospect, and makes his comeback in front of a much smaller crowd.
These guys are athletes. A lot of them are so competitive, it ruins their lives. I'm sure they're not making him do something he is opposed to.