GQ has published a long feature on Mookie Betts, written by Sam Schube, in which Betts talks, among other things, about leaving the Red Sox:
Early on Betts determined that he would turn down whatever contract extension he was offered in order to make it to free agency, where he'd be able to earn something closer to his true market value. He just as soon would have re-signed in Boston, he says—but only if they made the right offer.
Just like learning to lay off outer-half curveballs, turning down big dollars took practice.