I haven't seen any of the third season yet (for multiple reasons). So Unless they mention something there, I have only knowledge of the first two netflix seasons.
With that said, knowing the professor there may be something more than just getting complacency...
The main obvious point of them separating the groups was one, to get extra labor, and two lock the others up who don't trust the robbers.
They offer to 'securely' get them the money, so it won't get taken by the police.
Nobody at this point is thinking about if they'll actually get the money, they're thinking about surviving the ordeal, and maybe they'll get money. These are the 'weaker' individuals the Professor is taking advantage of. I believe the saying goes something like 'more flies with honey'...
While the the other group gets taken away. At this point my memory is vague if they just get locked up or get used for a different labour task anyways...but its unimportant to the question.
The hostages are in a pretty messed up situation and are under a lot of stress.
The Professor is playing psychological warfare to get what he wants.