in exchange for them showing up for work, and if the employee then shows up for work with an expectation of receiving what you promised, then you must give them what you promised.

Maybe you change the promise later and the employee can then decide whether or not to show up the next day.

But if you banked a bunch of vacation days under the old deal, they can't take it away by changing the policy later. The new policy applies once it is announced. Employers don't get to pretend they have invented time travel.