It started off with simple favors, then bribes (i.e. whiskey, basketball tickets), and finally money. Eventually they asked him to leave off whole flights completely, prompting Jake to ask for Nate Westen's help. Nate made a mess of it and dumped it on Michael.
Michael posing as "Steve Remmington", a fellow gunrunner, eventually helped Jake stealing all of the Zamar's manifest in the stash house.