Any leave you "sold back "should be already be included on your DFAS Form W-2, to include the amount of tax withheld. Just enter the W-2 in the W-2 interviewSelling Back Leave
Military members are authorized to get a cash payment for any unused leave when they separate from the military if they are retiring or separating with an honorable discharge.
You are authorized 1/30 of your basic pay for each day of leave you sell back. Depending on your paygrade and dependent status you may be authorized payment for a portion of your Basic Allowance for Subsistence (BAS), and a portion of your Basic Allowance for Housing (BAH) as well.
Members are entitled to sell back a maximum of 60 days leave during their career, you can sell back leave any time you reenlist, extend an enlistment, or when you are discharged.
Taxes are withheld from your final payment at the rate of 25% for federal tax and varying amounts for state tax, depending on your personal situation, on the basic pay portion alone.