I find that 5e D&D definitely seems to work best overall with a pre-3e approach to encounters - let the players with the 9th level PCs determine if they want to face the Balor and don't worry too much about what threat level it's supposed to be. XP - by-the-book generally works; it can be cut down if ridiculously excessive. BTB a Balor is worth 22,000 xp. Deadly for a 9th level party is 2400 each. You could halve the Balor xp award to bring it more into line with actual threat level (making it more like CR 14). Or you could just use the pg 82 chart and say 'this was a deadly encounter for my level 9 group, 2400 xp each'. Personally for my online 5e game though I'd probably just give full xp; maybe I wouldn't give much or any non-combat xp that session, and it'd soon
I generally find the main issue with 5e is to give reasonable xp for sessions
where the PCs are doing important stuff but there is no or trivial combat. Something like an easy encounter's worth per hour of play or medium encounter award per achievement (eg looting a treasure hoard) works well.