Your paycheck on terminal leave should be the same as it was before (I.e. you get BAS/BAH flight pay etc). When you sell leave back you only get your base pay which is why most people say its a bad deal but each situation is unique based on what you do or don't have lined up. The 25% tax is an IRS rule but you will probably get some of that back when you file taxes for that year based on your other income and deductions.