An easy way to get this in one season (doing 6 successful trades every 5 weeks) is to have YOURSELF offer a superstar, and have the OTHER brand offer up 1 dollar. The CPU will NOT say no.
What I usually did on week 5, week 10, etc (before the PPV) was to hire superstars on 2 week contracts just so I can have people to trade.
I believe you can also trade large amounts of money (like have yourself offer $100,000 and only ask for $1) but I haven’t tested this and I believe going the superstar route is cheaper.