As a new feature. The person initiating the duel sets the amount (let's say 50 gold) the challenger can either accept the duel or decline. If they accept and win the duel they get 50 gold from the other person, and, of course, the same goes for the other person. If they decline, well, nothing happens. They just don't duel. If the player has insufficient funds it will not let you accept.
Of course, you can always manually make an agreement with the current system, but you run the risk of people simply not paying up if you win.
If it were allowed cross realm I can see potential for exploitation. Though if there are rules applied like no assistance from outside, no one out side the duelers can neither attack them nor heal them it would be fun. But obviously some classes would be dominant while others wouldn't stand a chance.