Under cross-examination, the agent described how he and Chiheb Esseghaier had counted out $250,000 in a New York apartment and put it in the case, ostensibly bound for overseas.
Despite the agent's protestations, the normally frugal Esseghaier insisted in contributing to the stash.
"He was careful with my money even more than with his own," the agent said.
Ontario Superior Court also heard how a seemingly chance meeting between Esseghaier and the agent in fact came about as part of an ongoing anti-terror investigation.
"You were tasked to form a relationship with him?" Norris asked.
"That's correct," the agent said after an initial reluctance to answer.
Court heard Esseghaier, 30, was involved in an altercation over seating at the Houston airport before a flight to California in June 2012, when the agent befriended him.
They ended up as seat mates for the almost four-hour flight and quickly hit it off, the agent posing as a wealthy businessman from New York.
In September 2012, the agent visited Esseghaier, a Tunisian doctoral student, in Montreal and told him how their meeting had given his life a purpose, court heard.
"You had been inspired to more than just send money overseas: You wanted to be involved in some action," Norris summed up what the agent told his new friend.
Esseghaier began opening up about having visited Iran, court heard. There, he said, he had met "The Responsible One," a man with money and apparently close ties to terrorist mastermind Osama bin Laden and Ayman al-Zawahiri, who succeeded bin Laden as head of al-Qaida.
The "laser focused" Esseghaier, the agent said, talked about having received training, including how to recruit people to carry out terror plots such as attacking the train, poisoning soldiers on a military base, or attacking non-religious Muslims.
It wasn't exactly good training given that he ended up recruiting an undercover FBI agent, Norris noted.
"That would be viewed as a snafu," the agent agreed.
The agent also testified Esseghaier appeared to identify "The Responsible One" as a man who helped in the hijacking of an American plane in Pakistan in 1986 in which at least 20 people died. Norris, however, cast doubt on the identification.
Norris represents Esseghaier's co-accused, Raed Jaser, 35, a Palestinian dispatcher from Markham, Ont.
Esseghaier, who does not have a lawyer, earlier waived his right to cross-examine the agent, sitting motionless when the judge asked if he had any questions.
During the undercover agent's two weeks on the stand, court has heard about the alleged plot to attack the passenger train between New York and Toronto, much of it in hours of secret audio recordings.
At one point, Esseghaier is heard telling the agent he wants to see Islamic Sharia law imposed everywhere.
"I am making my obligation of jihad," he says. "I believe in it, but not just by words, but also by action."
Justice Michael Code warned jurors he would be giving them "complex" instructions on what use they should make of the wiretaps, which in essence are hearsay.
In all, the agent said he likely spent thousands of hours with Esseghaier, but only about two weeks with Jaser, who withdrew from the alleged train plot.
"He got scared" Esseghaier is heard saying derisively on one intercept. "He was scared of the police."
Jaser and Esseghaier face multiple terror-related charges in the alleged plan to attack a train travelling from New York to Toronto. Not-guilty pleas have been entered for both men, who were arrested in April 2013.
Their trial continues Tuesday.