In 2015 I was overpaid by error by employer. They corrected this, but only in 2016. I wrote a check for the gross sum. They said I need to claim the federal taxes back in the 2016 tax year. How do I do this?
This situation is referred to as a Claim of Right under IRC Section 1341. It can be claimed as either a miscellaneous deduction or as a credit (if the amount repaid is more than $3000). The deduction is the simpler way to claim your repayment, but a credit might save you more money.