If the entire goal was to beat a person up, I am not exactly sure why they went through all the steps. First of all, they took the risk of their phone not being returned, PERIOD. Then they gave the woman a cash reward, only to take it back? Something doesn't sound right about this story to me.
Sounds like they offered the reward because they thought it would never be returned without one, and had decided in advance to take it back. The lady likely would have returned it without a reward. Her only chance at not being beat up by the thug couple was to refuse the reward. (course she had no way of knowing that, they handed her cash ..)
Since she had their contact information from the phone it shouldn't be too difficult to locate them and file a police report and send them off to jail for assault and theft. They gave the cash to her and then stole it back.