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, and the main point of them separating the groups in general 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're 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. 

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.