You should be able to lower your ProN and flip the switch to mark that investment as bought with a save editor that can display the game's switches(investments) and variables(ProN).
I don't think anything else but that one switch gets flipped when you bribe the arigarda, and it doesn't get checked before the battle of yhilin so if you haven't done the battle yet that should be all you need to change.
It should be switch 740 Arigarda II
for the bribe investment