It's awkwardly written definitely, and I'm not sure I've parsed it correctly either but:

tipsters in with a chance

I read as 'tipsters who have a chance'. 'in with a chance' is the phrase here that sits awkwardly. It appears another verb has been elided and adding one back in e.g. "Tipsters who have a chance of receiving the FBI portion..." would be more complete and easier to read imo but it's still a mess of a sentence.

As for the self-nomination, it's addressed in the next paragraph

This means the McDonald's worker will have to be put forward by an investigating agency, such as the Department of Defense or the FBI, which is then reviewed by an interagency committee