Dispose of merchant

 is a quest given by Decker to kill the merchant Daren Hightower and his wife for refusing to deal with the other caravans and the Underground.

Walkthrough
Collect your $500 down-payment from Kane, then go to the southwest exit grid in Downtown of the Hub. This will take you to the Heights. The Hightowers live in the building smack dab in the middle of the section. Talk to the guard outside the door, and mention that you're running late for an appointment to get into the mansion.

Then, just walk to the bedroom and start attacking. Both of them must be killed, and talking to Mr. Hightower will result in you being kicked out, so just shoot them. All 5 of the guards will come running, armed moderately well. Three have 10mm SMGs, one has a Desert Eagle, and Leon has a 14mm Pistol. Decker suggests eliminating witnesses, so kill all of them too.

Another way to get the quest done is to come at night, after 7PM, and say you're going to go away. Then, go round the house clockwise, but don't turn the corner so that the guard in front of the door doesn't see you. When he himself turns the clockwise corner, run to the door, open it, then close it once your companions are in too, and before the guard returns. You can now kill Hightower and his wife without the guards minding, as long as you end combat mode before you go out, and you doesn't get in their sight on your way to the exit grid (follow your way back to do so). There isn't any particular advantage for doing so, except if you just want to avoid bloodshed or are weak in combat.

You can then head right on back to the Maltese Falcon and speak with Kane to get your reward. Don't worry about retaliation from the Hub Cops, they will not turn hostile.

Take down Decker
After getting this quest from Decker, you can choose not to do it at any time and rat him out to the Hub Cops. They'll request your help in raiding the Maltese Falcon and stamping out the Underground. You can even do this after completing the quest and receiving your reward.

Rewards

 * 500 XP
 * 2000 caps
 * Decker's next quest: Dispose of Jain

Thieves' Guild
There is a locked footlocker titled "Strongbox" inside the mansion; inside is the Necklace that Loxley wants stolen. Since you're blowing apart the Heights anyway, now is a good time to retrieve it. Be careful though, the box is trapped!