In theory, you should only pay back 92.35% of the bonus. This is because, your employer can get the 7.65% withheld for FICA (social security and Medicare tax) back from the Social Security Agency. But, in practice employers usually ask for the full amount back, either because they're not aware or don't want to do the paper work, including written consent from you.
Actually, they can get back 15.3%; the 7.65% you paid and the 7.65% they paid.
