The best solution would be to get the broker to amend the form 1099-B, but that may not be possible.
You should report the sale in 2020 since that is when you received the proceeds. In 2021, you will need to enter the sale as you normally would, just modify the cost basis to cancel out the sale amount.
Another option is to not report the sale in 2021, but you will get a notice from the IRS assessing additional tax. If that happens, you can explain what happened and the IRS will reverse the assessment.
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