@dequbed hmmm... ok. Maybe tell them that, and ask for a consultancy fee? (If you're legally set up for this sort of thing.) Most corpos won't even notice costs that are less than five figures.
Bonus: This might be a PITA in terms of paperwork. But once you're set up as an established "vendor" in their system, it'll always be easier for them to go to you for this kind of work than to anybody else.