Having a little fun with google, it appears Wade Boggs did not play the last 4 games with a questionably pulled muscle to edge Mattingly for the batting title in 86.
Ty Cobb may have sat out a few games to protect batting title leads 100 years or so ago. From this website, I have no idea how credible.
http://www.nakedwhiz.com/bbjcobb.htm
I suspect this has happened more than just this year.
In the NBA, I think David Thompson one year had to score 70 points to win the scoring championship, so teammates were instructed to not shoot.
Pro sports is a stat driven business, and achievements like this pay off with millions.