doesn't seem exactly right since it should be gross pay less social security and medicare taxes. the employer gets them back from the iRS.
as to income tax withholding, say you got a $20K bonus on which there was $5K federal withholding ignoring social security and medicare your net was $15K if the actual tax on the $20K was only $4K you would have gotten back $1K from the IRS. so now you have $16K
under IRC 1341 if the repayment is more than $3K you can go back to the year you received the bonus and recalculate the tax without it. the decrease in tax of $4K is taken as a tax credit in the year of repayment so you would get back an extra $4K total you got $20K total you pay the employer $20K
