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 blasting! 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, packing significant heat. 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.

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 don't seem to mind.

Reward
2000 caps, 500 XP, and Decker's next quest; Dispose of Jain.

Take down Decker
After getting this quest from Decker, you can chicken out 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.

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!