assuming repayment was in a subsequent year and say the bonus you got in the first year was $10K you should have repaid $10K less the medicare and fica taxes. on the original return, you got credit for the federal income taxes actually withheld and that has not changed since you do not file an amended return for that year.
in year 2 you (the repayment has to be more than $3,000 to get your income taxes back) :
a) recompute the income taxes for the year you got the bonus but without including the bonus in box 1 of the W-2
2) you take as a credit on schedule 3 line 12d denote as IRC 1341 the reduction in income taxes in year 1
how all this works - simplified
example 1
say in the bonus year there was $4,000 in federal income tax withholding and that's the taxes you owed on it
you got a net check of $10K less $4K less Fica and Medicare taxes of $765 or $5235 (in pocket).
you repaid your employer $9235 ($10K less Fica and Medicare taxes). the return for the year of repayment will show a credit of $4000. so your net out-of-pocket is $5235
example 2
say in the bonus year there was $4,000 in federal income tax withholding and but you only owed $3,000 in federal income taxes on it. you got a net check of $10K less $4K less Fica and Medicare taxes of $765 or $5235 (in pocket). the employer withheld $4K but you got $1K back from the IRS because there was over-withholding. so you are in pocket $6235.
you repaid your employer $9235 ($10K less Fica and Medicare taxes). the return for the year of repayment will show a credit of $3000. so your net out-of-pocket is $6235
state laws differ
you are not required to take a credit in the year of repayment
you could take it s an itemized deduction on schedule A line 16 denote as IRC 1341 repayment
