@Tuckerlee there are two choices
1) easy method, simply take an itemized deduction for $6,000 on the 2023 tax return. This is only beneficial if you were going to itemized anyway.
2) determine how much your tax would have changed had your earlier tax retirn not included the $6,000 signing bonus. Whatever that result is can be taken as a tax credit on your 2023 return.
3) you can NOT simply amend the prior year tax return.
