You're right, crafting a contract that prevents her from doing collateral damage is probably more trouble than it is worth. But, my suggestion isn't to save her. It's to use her considerable power to kill even more demons in places where you are not.

If you're really willing to go for high risk/high reward, you let her keep the scales and command her to turn every demon with less mana than her into a suicidal demon-slayer themselves. Frieren gets to continue on her casual journey while more and more demons wipe out their own kind.

Although that tactic sounds a little too much like western universities now that I think about it...