Regarding when the FBI came to know about the dossier, according to Sharyl Attkisson's
time line:
July 5, 2016: ... Fusion GPS’ Steele, an ex-British spy, approaches FBI with allegations against Trump, according to Congressional investigators. Days after closing Clinton case, FBI official Peter Strzok signs document opening FBI probe into Trump-Russia collusion.
and
Summer 2016: [this entry falls between July 31 and Aug 4 in her time line] ... Fusion GPS’ Christopher Steele, a British citizen, briefs FBI leadership on his anti-Trump political opposition research. Weeks later, the info makes it to FBI agent Peter “Strzok and his team,” according to New York Times.
and
Mid-to-late Sept. 2016: Fusion GPS’ Christopher Steele’s FBI contact tells him the agency wants to see his opposition research “right away” and offers to pay him $50,000, according to the New York Times, for solid corroboration of his salacious, unverified claims. Steele flies to Rome, Italy to meet with FBI and provide a “full briefing.”
Dirty, dirty people.