Steele was already working with the FBI in February 2016
From the FBI Vault today
Records that he was 'admonished' by the FBI for revealing their relationship to a third party
The dump also contains lots of redacted receipts, indicating they were paying him for a long time.
https://vault.fbi.gov/records-between-fbi-and-christopher-steele/records-between-fbi-and-christopher-steele-part-01-of-01/view