IRS Publication 525 says the following.
"Repaid wages subject to social security and Medicare taxes. If you had to repay an amount that you included in your wages or compensation in an earlier year on which social security, Medicare, or tier 1 RRTA taxes were paid, ask your employer to refund the excess amount to you. If the employer refuses to refund the taxes, ask for a statement indicating the amount of the overcollection to support your claim. File a claim for refund using Form 843.
