I agree with your interpretation, I think it’s the right one.
I’ve edited in 斎藤の返事🔝.
To which Unibrow (でくねにや）replies:
Click to reveal
➃《だったらまた当直『の』バイトやれよ》
だったら: adv. n. if that’s the case
➄《一晩『で』8万ももらえるんだろ？》
If that’s the case [that you’re running out of money] you can do your part time job at 80,000 円 for one night.
“A” in today’s set begins right after Unibrow has made the suggestion for Saitou to return to his part-time job.