Errors in Withholding
Occasionally, a nonresident alien’s employer withholds Social Security and Medicare taxes from pay that is not subject to the taxes. When this happens, the nonresident alien employee should first contact that employer for reimbursement. The employer will amend their employment tax return(s) to get back the employee and employer Social Security and Medicare taxes paid to the IRS.
Then, if the employer does not refund the withheld taxes, file a refund claim on Form 843, Claim for Refund and Request for Abatement.