Retirement tax questions

Form what you said, my understanding is that you received a request to pay back the benefits in 2020 and you did so in 2020. So, the fact that the IRS cashed the check in 2021 would not affect the treatment of the repayment belonging to 2020. So, you should treat the repayment as belonging in 2020.

The repayment amounts need to be applied to the years in which the original benefits were paid. For 2020, you would not report any social security benefits on your tax return if you repaid all of the benefits you received. You should request that the social security administration correct your SSA-1099 form to reflect the repayment in 2020. If they will not do so, you should mail in your tax return and attach an explanation as to why you are not including the social security income on it.

You will have to recalculate the income tax in prior years and enter a credit on your 2020 tax return to get credit for taxes paid on the benefits in prior years. Here is something from IRS publication 915 on how to do that: