Helping my mother in law with her taxes and noticed her employer had different gross wages for social security/Medicare on her w2. She is an aide to an elderly woman and they do not take out any pretax deductions. The bookkeeping firm that processes them is saying they are "charging back the social security and Medicare withholdings" to her which is reflected in box 1. I have never heard of this before (owned a company so know how corporate/employee taxes work). Is this something an employer can do?

As an example, say she earned $10,000 and they report $620 as SSI withheld and $145 For mc withheld. However, they are reporting gross wages as $10765.

If the employer withheld social security, or Medicare taxes less than the correct amount from an employee's wages, he can make it up from later pay to that employee. But he is the one who owes the underpayment. Reimbursement is a matter for settlement between he and the employee. If, for example, the employer deducts less than the correct amount of FICA tax, he is nevertheless liable for the correct amount of the tax. Until collected from him the employee also is liable for the employee tax with respect to all the wages received by him.?

The employer withheld the correct amounts for the earnings she actually made. I understand that if it was under withheld and what not that is owed, but that's not the case here.

The issue is that they are increasing her gross wages reported in box 1 of the W2 by adding the employer's match to it of the social security and Medicare withholdings. I guess they are running it as a benefit?? But that doesn't sound right. So the employer pays in the match, but then "charges" it back to her by adding it to gross wages on the w2 box 1 only. Her actual wages are correct in boxes 3 & 5 as matched with her paychecks for the year, the increase is only box 1.