US Postal Service writes a check for any unused vacation time and we can carry over 55 days to the next year.   Accumulated sick days just get added to the service time towards the pension.

Edit to say that USPS advances you a years worth of vacation days on Jan 5, but if you retire before the end of the year they take back the days you haven't earned.    So you can't retire the day after the days are awarded and expect to get paid for them.