We don't know and it's not explained.
In both the draft script (where it's not mentioned at all), the final revised draft script and the novelisation it's simply described as a "plan", one that has something to do with Marty's access card.
‘Hey, Needles,' Marty replied, trying not to sound nervous.
Needles didn't seem to notice. Instead, he launched quickly into
exactly what Marty had to do if their little plan was going to work.
Somehow, whenever Needles explained this sort of thing - in all the
years they had known each other, ever since high school -it always
sounded so easy.
So what do you say, Marty?' he finished breezily, once more flashing
the grin that had gotten Needles to a position in the organization
that Marty could never hope to reach.
Back to the Future Part II novelization
According to the usually highly accurate KristenSheley.com website, there was a brief mention in a revised draft script that it was a "902 deal" but beyond that, no further description. Note that the order of events play out slightly differently.
Inside the McFly's 2015 house, things play out similar as they do in
the film, though there is more conversation between Lorraine and
George. We learn, for example, that Marty blamed everyone but himself
for the race with the Rolls Royce. When Marty arrives home from work,
he takes a message from a "Harrison" at work, who tries to cajol him
into the "902 deal" at work. Marty is reluctant, as he is up for a
promotion, but the chicken thing provokes him. Fujitsu pages him
during dinner to fire him, as Harrison turned Marty in after
"catching" his illegal transfer. Even when Marty explaines he was
tricked, his boss still says he's fired "for being stupid enough to
fall for it."
THE FUTURE THAT NEVER WAS....