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You can activate this quest by either talking to the old man in the Red Dragon restaurant or the old man running the fortune telling booth outside the Golden Temple. Apparently these two were mafia hitmen back in the day and did a job together years ago. The job was highly profitable and the money was stored in a cash box, with each of them taking a key needed to open it up.
Neither of the men trusts the other one. Both Ji Wen Ja and Lu Feng tell you that they want you to kill the other and steal their key to the cash box. You're initially offered 10% for the job, but when you to speak to the other man, he will offer you a higher 20% cut to kill the original guy you talked to. If you keep bouncing back and forth between the shady old men, you will eventually be offered an 80% share of the loot. At this point, you can accept and kill the other to be rewarded $1000 in cash and four points of Experience.
However, if you use a little Persuasion, you can convince the old friends to reconcile their differences. Although this won't get you any cash, you will be awarded an astounding six points of Experience and a point of Humanity. On top of that, when you revisit the other old man, you'll receive yet another point of Humanity.
Another way to handle the situation is to bid the two of them against each other until you've earned a 50% cut, where you should get the option to use Persuasion on Ji Wen Ja (if your skill is high enough) to talk him out of the assassination attempt. Instead of choosing that option, though, return to the Red Dragon and kill Lu Fang for four points of Experience. Since Ji Wen Ja doesn't know that Lu Fang is dead, return to him and make use of the Persuasion option to make friends with the now-deceased Lu Fang. This will earn you an additional six points of Experience, for a total of ten points for the whole quest.