My employer will not pay us for accrued leave. I saw the end result recently when a co-worker took a week and a half off only to return with his resignation in hand. Guess he cashed out his vacation time anyway....
Now we all realize that if someone is going to resign, it's most likely going to happen after a long vacation. I don't know if HR realizes this yet.
(not a lawyer) I think in the US (oops, probably should have said California) generally accrued vacation has to be paid off (as it is accounted for at the time it is earned, but there may also be a cap which causes some to disappear). But sick days are not generally considered a benefit and avoid this. I imagine "flex time" is more complicated.
Usually depends on which state you are in. California I believe they are required to pay up to a certain number of hours.
In CA leave is vested, it has to be paid out. Not all states have this policy.