Can anyone please teach me what this phrase "work a system of favors" mean?

In the novel I'm reading now, a hero is blackmailing a heroine, who is an expert safecracker, to crack a safe which has important documents the hero is looking for. He doesn't tell her what kind of documents they are, and she is so scared to take a risk and asks if the documents are worth risking their lives.

And here's the thought that goes around in his mind in response to her question:

> 
> It was so much easier to ask a question like that from the outside of a mission, when you hadn't established the relationships, worked a system of favors, laid the groundwork.
> 
> 
> 

I've looked for the meaning of "a system of favors" and guessed it would mean "a form of exchanging favors" (and please correct me if I'm mistaken), but the word "favors" sort of pops up in this paragraph, and nobody is talking about doing or exchanging favors even in this chapter, so I can't grasp its context and nuance.

Also, is "a system of favors" a thing you work? Can it be rephrased as "established a system of favors"?