the taxable wages on the W-2c SHOULD NOT change. thus your taxable income will not change for 2020. the taxable FICA wages and withholding may change depending on whether or not the overpayment reduces the taxable amount and thus the corresponding withholding. Medicare wages and withholding should drop.
you can not claim excess FICA withholding from one employer on your tax return. the rule for that is you must get the money from your employer. There really isn't n excess medicain other words if your gross bonus was $10,000 and the amount was subject to FICA and MEDICARE you likely should only have repaid
$10,000 less FICA of $620 less Medicare of $145 net $9235.
the company files 941 in 2021 with IRS claiming credit reduced wages of $10,000 which gets them back the portion of FICA and medicare that they pay of your bonus + the amount withheld from your bonus.
if you repaid the Gross they owe you the FICA and Medicare taxes taken out.
you'll have to wait for the w-2C to see what has been changed. only then can it be determined if they handled the repayment properly
in 2021 you use section 1341 to claim the credit.
I do not know what NY law says about these repayments.
