Given that a character can cast Planar Ally, what is the highest CR they can summon?
Can they summon a monster with a CR higher than their own level?
Would a level 11 cleric be able to summon a CR 19 Balor and make a deal with it?
There is no limit, yes, and yes.
But you the Cleric cannot choose what will come. Per the text of Planar ally:
That entity sends a celestial, an elemental, or a fiend loyal to it to aid you, making the creature appear in an unoccupied space within range. If you know a specific creature's name, you can speak that name when you cast this spell to request that creature, though you might get a different creature anyway (DM's choice).
There is no limitation to the creature's CR of any kind. The DM will most probably try to make it fit the scenario and fit your relationship with the otherwordly entity. Note though that the summoned creature...
is under no compulsion to behave in any particular way. You can ask the creature to perform a service in exchange for payment, but it isn't obliged to do so.
...although it is sent to primarily aid you:
That entity sends a celestial, an elemental, or a fiend loyal to it to aid you
Still, you should pray to your DM right after your Cleric finishes praying for a planar ally.