The easiest solution would have been if the employer corrected the W-2.
According to the IRS, they should have done that.
In TurboTax, you can go to
Deductions & Credits
Other Deductible Expenses
Continue through this section and enter it as "Claim of right repayment over $3,000"
This is really for when the repayment is made the following year, since, as I say, when done the same year the employer is supposed to issue a corrected W-2C.
There may also be an issue if Social Security and Medicare tax was withheld and paid on the inflated wages.
This will put the repayment on Schedule A line 16, so it's only advantageous if you itemize deductions, but it will lower your taxable income.
Your other option would be to file a complaint against the former employer with the IRS.
You might also need to file Form 843 to get the excess FICA tax refunded.
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