In The Fast and the Furious: Tokyo Drift, we find that Han is in some sort of business with Yakuzas and is actively ripping them off (even befriended the Yakuza's nephew to gain their trust etc). He doesn't appear to be stealing very large sums of money though. Chronologically, Tokyo Drift takes place after Fast & Furious 6, which happens after Fast Five after which all of the team became filthy rich (enough to buy private jets, multiple Lamborghinis and what not).
Further, in Tokyo Drift, Han himself claims and demonstrates that he has plenty of money. He can't be stealing significant enough amounts to be comparable to what he already has. He probably doesn't even need to be in business with the Yakuza anyway, since he has enough money to last more than a lifetime. Then why is he doing it and, more importantly, why is he stealing insignificant amounts? Just for fun?