So, doing MNF will forfeit him entirely from doing any MLB games for FOX? Is he allowed to work for both networks?
I'm not sure. Sports announcers and game analysts are actually independent contractors; they do their work for and across different broadcast entities.
Take basketball for example. Mike Breen is the No. 1 announcer for the NBA on ESPN - but he also does New York Knicks games for the MSG (Madison Square Garden) Network.
And when the NCAA Men's Basketball Tournament comes around, TNT NBA studio analyst Charles Barkley goes over to CBS to offer his commentary on the collegiate game matchups throughout "March Madness".
Since it's they are different professional sports - and their seasons overlap only two months- September and October (include the first week of November with the expanded MLB playoffs).... I don’t see any problem with Joe Buck continuing his baseball broadcast career with FOX, while going to ESPN to do MNF.