FBI informants protected from prosecution

HERE'S A SCENARIO...

The FBI has been known to get charges, sometimes very serious charges, dropped against informants who are giving them valuable information in their investigations.

Lets say that the FBI has a low level mob informant who is supplying good info on a crime family. The informant gets busted on a serious charge but they don't want him serving time. How do they pull strings with the police department and keep their rat out of jail without tipping off "bad cops" who are in bed with mobsters?

I'm guessing that they would never go directly to the cops who are building the case and say, "Lay off our informant." They'd use another route, like the District Attorney or an Assistant DA that they know they can trust, who would make sure that the case isn't prosecuted or that a sweetheart plea deal is made.

Do you think I'm right? And what other scenarios are there?

One give away that someone is an informant would be that they get busted and then mysteriously get off somehow.

Greg Scarpa was suspected of being a rat way back decades ago, he was getting arrested by New York police with his partners, he kept getting off. His son spoke of it in his book. Perscio trusted him while being ratted out by Scarpa on a frequent basis.

Where is the report link?

HOUSE REPORT: THE FBI'S USE OF MURDERERS AS INFORMANTS