It doesn’t necessarily work that way with the retention bonus. Athletic departments have different pots of money. Ticket $, media rights $, student activity fee $, marketing rights $, general donation $, school capital project $, concession $, specific donation $, and more. There are different rules for how each pot of $ can be spent, some more restrictive than others.
In college athletics it is usually just the coach’s base salary that is paid out of general athletic department funds which would be mostly from ticket dollars and media rights dollars. Things like retention bonuses or annuities are usually paid by boosters, often even specific boosters. Aka Booster Billy Bob signs an agreement to donate $400k on April 30, 2021 with the legal requirement that it go to Pitino’s retention bonus. If it’s not going to the retention bonus then Billy Bob is off the hook and doesn’t pay a dime. Now, maybe Billy Bob has decided he doesn’t really like Pitino so he’s ok giving the $400k and having it go toward the buyout, but maybe not.
My point is that it’s not a given those dollars are still available. At least not without doing some schmoozing to convince Billy Bob to change the requirement on his donation, or maybe finding another booster to cover it.
Money from different pots can be used to cover other expenses so there can be some creative work arounds with a good finance person, but donated dollars are usually among the more restricted.