A devilish FoP could use its abilities to tempt people into giving up their soul. For example, it could use Magic Item to buff someone's equipment, or Ally or Enemy to buff their stats in exchange for performing Evil actions.
It would not be allowed to possess a person and use Control Creature to compel someone to give up their soul, or even use Curse Item or the "Enemy" debuff of Ally or Enemy to penalize someone until they "willingly" gave up their soul.
Devils give you things, and then threaten to take them away unless you serve them.