Exactly what I came here to say. Many if not most informants are people who cut a deal in order to get out of their own charges because the evidence against them is so strong that they wouldn't stand a chance in court.

Now it is possible that the FBI had him on financial crimes and used him to get to Epstein however that's unlikely the case considering the courts have found Trump liable for one sexual assault (E. Jean Carroll) and he's been accused of several more including the rape of a 13 year old: https://www.politico.com/f/?id=00000158-26b6-dda3-afd8-b6fe46f40000