Tracking Social Security Income Benefit in HCM

We have few employees who receive Social Security Income and would like to forfeit bonus because that would put them above the SS income limit from other sources. We would like to track this (Employees that want to forfeit their bonus) in HCM and thinking of a flex field. Mostly this would be used only for tracking purposes and has no other impacts. Did anyone face this requirement and if so, any thoughts on how to handle this.