From billr:
It’s obviously Tax Season (in the US), but I don’t see any joke here. All I can think of is, “floating a balloon” as a financial thing, but aside from that, nothing.
All I get from my accountant is a piece of chocolate (and a bill)!
A Balloon?!
Comics I Don't Understand 
I think it’s just a simple absurdity joke. Normal event + absurd outcome = comedy. It’s just that some absurdities are funnier (see Monty Python for many examples).
I think it’s a variant on the “child goes to the doctor and gets a piece of candy (or a balloon) for being brave” thing.
Yeah, I like beckoningchasm’s take. Balloon as a prize for putting up with the ordeal.
Although I think I would have liked it better if the preparer didn’t look so disappointed.