Generally, if there is no agreement to the contrary defining the exact repayment amount, you would owe the full, pre-tax or gross amount you received, since that is what was paid both to you and for you (e.g. in the form of tax payments). You would then have to make the appropriate adjustments on your own taxes, such as by filing an amended tax return for a prior year on which you were taxed on the bonus, so as to get a refund.