The best way I can think of is to go back to Balamb Garden, and play someone there. But when they say
"Let's play with FH's rules or Timber's rules too" or something similar keep choosing "no".
Eventually he/she will just say ask to play cards without introducing other rules. Now playing in Balamb, the trade rule is always "One", so play someone there and win once or twice. I'm pretty sure that would give "Direct" the shoulder and you should now have trade rule "One" or even "Diff."

TIP: In case you didn't know, you can do this process of saying "no" to everybody in FFVIII so you just play with Balamb's rules. But places like in up in space or Centra you can't.

Anyway try that, it has worked for me before although I don't know the correct way of distributing rules.