​Best way to compensate an employee that does NOT count towards income limit for SSDI?

Basically title. I have a part-time employee that I want to promote, however, this employee is currently on SSDI and can only earn up to a specific amount before her benefits are negatively impacted… Problem is, we already pay her close to this limit.

I would just throw her something extra off record, but it would bug me knowing I couldn’t really claim the expense without doing some “colorful” bookkeeping. I’m just wondering if there’s any sort of fringe benefit I can offer to this employee that would not count as “income” for SSDI?