The government agent was a mysterious figure who served in President Richard Nixon's administration.
In the alternate timeline where an elderly Biff Tannen traveled from 2015 to 1955 to give his younger self a sports almanac to make himself rich, the government agent approached Biff in 1972. This occurred just as Jayne Mansfield, Biff's second wife, left him after discovering a secret fallout shelter beneath Tannen Manor that housed pictures and sculptures of Lorraine Baines McFly, the woman Biff was obsessed with since high school.
The agent told Biff that he owed taxes, but that he'd never have to spend any money on taxes ever again if he did business with Nixon. After the agent told Biff that the money and power he could earn would win him Lorraine, Biff agreed. He set up BiffCo, which was used to hide any of Nixon's shady deals, and give the president deniability. In turn, Biff used it to purchase up all of the land in Hill Valley.
In 1973, BiffCo purchased The Washington Post, due to the fact that they owned the Hill Valley Telegraph, which published an article critical of BiffCo. Biff renamed it The Biffington Post, and fired anyone who made articles that he didn't understand.
Thus, when Bob Woodward and Carl Bernstein sought Biff's approval to continue working on the Watergate scandal, Biff fired them. Nixon himself thanked Biff for stopping the articles about Watergate. Biff asked for help running the paper, and Nixon told him that he'd send his own men there to work on the paper to keep Biff's hands clean (although Biff thought he was talking about the ink on the paper).
Afterward, when Biff stated that he liked Nixon and wished he could be president forever, the government agent informed Biff about the 22nd amendment of the United States Constitution, which forbade a president from holding more than two four year terms. Biff began to bet on sporting events in order to gain money to bribe state legislators to repeal the 22nd amendment, which allowed Nixon to win three more terms in 1976, 1980, and 1984.