Did Akira just get roped into fighting another bounty monster for no pay at all?
Opposite. The point here is that given the unexpected challenge and in turn extra expenses (lots of extra missiles, wasted beacons etc), along with Akira and "Nergo" performing far beyond expectations, the amount of bounty (800m) isn't enough to both treat them squarely and cover everyone else and expense. Shikarabe and co aren't after the money at all, and even if it's not an official contract they want to treat everyone fairly both for their own reasons and for purely practical ones that reputation matters a lot and they want to keep extremely well performing and unaligned hunters friendly to them going forward. However, they can't afford to just give away hundreds of millions themselves either. Nergo they're not too worried about, because he too is not after money (as readers we can see he clearly has his own agenda even below the cover agenda of connections, but the cover agenda is still cover). But Akira is 100% about the money, though he's not such a putz he'd totally ignore long term interests or be unrealistic about expenses either. He'd still feel negative about not getting a proper reward however.
So now Shikarabe semi-covertly taking another high paying job, except this time he's not in charge of any major expenses at all. It looks like he's basically planning on making up the gap through simple hard work: he can get a 4 man cut of bounty for this other monster, then once again choose to give his up and use it to pay Akira. If it works everything is square.
As far as I'm aware, the contract he made only covers fighting that one bounty monster and he's not in the contract that's been made to provide extra help.
As was stated directly in the negotiations, the "contract" here is about honor/reputation and practical game theory and such as much as anything, it's outside the hunter office system so it's not "legal" per se. Regardless of the exact warding, if Akira felt ripped off or jerked around, or even if he felt it just hadn't been worth the effort, that's going to negatively affect his attitude towards Shikarabe going forward. Shikarabe now is sure Akira is very strong, curious if he's got something special "more" about him as well (because somehow lots and lots of people who should know better don't recognize the strength, and while once is a coincidence and easy to ignore, for someone looking for it seeing it over and over and over again is going to start to raise questions), and also knows Akira isn't really aligned with anyone already (at least, in that sphere, obviously Alpha is in the background secretly, but it's true Akira has no deep loyalty and connection to a faction Shikarabe knows about). They also already have some level of working relationship. And finally Akira is very young: he's got potentially (from Shikarabe's POV) a long working life ahead during which he can get even stronger and potentially eventually rise to a serious level. That's a valuable combination, well worth going to some extra effort to nurture a connection even if it costs some money.
Edit: also forgot to add, don't forget in the background as well are Elena and Sara. Akira hasn't forged connections to any major faction yet, but he
has forged a few connections, and to a few pretty capable people that Shikarabe also respects and wants to keep a working relationship with. Remember back in chapter 61 before actually going to meet about the current job at all, Akira called them up, and Elena explicitly said "Also, let me know if he gave you a bad offer." They take a more than purely professional interest in Akira for obvious reasons, and that would also clearly extend to "let me know if he screwed you on the actual job".