Simple RAW for 5e V: The Disadvantage Strikes Back

[Hairfish]

Q398
say, a wizard uses Planar Ally and requests a Balor. He will ask Balor to ravage the settlement for five minutes, getting 500gp in return. Is this OK? Can wizard be provided with another demon? Lower CR? What are the limitations of Planar Ally?

thanks

A398

There is no RAW here. The outcome of this spell is almost entirely subject to DM fiat. The only guarantee is that you get a celestial, elemental, or fiend that's loyal to the entity you asked for help when you performed the spell.

The rules of the spell explicitly say the DM is under no obligation to give you the creature you want, nor for it to agree to any deal you propose.

That said, the guidelines provided in the spell's description do say that a creature that likes the task might agree to perform the task at a reduced rate or even for free and a non-hazardous task may also merit a discounted rate. Unless this is a particularly well-defended settlement, both of those considerations would probably apply to a balor.