Dealing with Contreras

 is an unmarked quest in Fallout: New Vegas.

Quick walkthrough

 * 1) Speak to either Lt. Boyd or Sgt. Contreras in Camp McCarran.
 * 2) Side with Lt. Boyd and gain NCR fame and This Machine. (Note: You can get info on Contreras and then kill him, and still 'turn him in' and obtain "This Machine". This allows you to sell your items to him, spend your cash on his special items, retrieve them from his body and still complete the quest.)
 * 3) Side with Sgt. Contreras, expand his inventory and receive This Machine.
 * 4) Side with Keller; neither Keller nor Contreras will reward you with This Machine for completing the quest in Keller's favor.

Siding with Lt. Boyd
You can start this quest by either speaking to Lt. Boyd in the Camp McCarran terminal or by proceeding directly to Sgt. Contreras' computer in the Camp McCarran supply shack and hacking it (Science 50). The Supply Shack is at the back of the terminal building via the Camp McCarran Concourse; enter the terminal, go up the escalators, down the hallway to the concourse. When in the concourse bear left down a ramp, and bear right to the door of the supply shack. Contreras's computer will be on the upper level of the supply shack.

Downloading the weapons manifest and returning it to Lt. Boyd will complete the quest. She will reward you with This Machine and you will receive NCR fame. By turning in Sgt. Contreras you will drastically decrease his weapon, ammo, and apparel reservoirs and he may be found in the far right un-openable holding cell. However, he can still be interacted with if you aim at him in V.A.T.S. first without attacking. Since he is one of the best weapon merchants in the game, and since the quest reward (the unique This Machine) is awarded to you regardless of whom you support, you should think carefully before siding with Lt. Boyd.

You can also complete this quest at any time if you have sided with Contreras or even Keller. If you downloaded Contreras' manifest, or ran his errands and noticed that his "medical supplies" could be used to make chems (Medicine 25 with Contreras or Blake), then you can bring either of these to Lt. Boyd at any time to complete the quest and receive This Machine. Sgt. Contreras will wind up in a prison cell as a result, and he will still act as a merchant though he won't restock like other merchants.

Siding with Sgt. Contreras
You must be level 12 to start this quest. You can start it by finding Sgt. Contreras in the Camp McCarran Supply Shack. The Supply Shack is at the back of the terminal building via the Camp McCarran Concourse. Enter the terminal, go up the escalators, down the hallway to the concourse. When in the concourse bear left to a down ramp, and bear right to the door of the supply shack.

Speak to Sgt. Contreras about his supply, and you'll have to pass a Speech/Barter check of 50 or pay 50 caps. Contreras will then ask you to speak to Isaac outside of Gun Runners.

Isaac should be located near or at the reloading bench to the right of the Vendortron at the Gun Runners storefront (If Isaac isn't there, he may be in his house directly to the east of Gun Runners, behind the Hostetler Home. If he isn't there either, check the Bugs section for this quest). Speak to Isaac and you'll either have to pass a Speech/Barter check of 80 or you can give him 100 pieces of junk metal (tin cans, hot plates, bent tin cans, metal pots, pots, pans and scrap metal). If you must scavenge the metal pieces yourself, the Aerotech Office Park (suite 200) has over 100 tin cans and bent tin cans. Once you've done this, you can return to Contreras.

Contreras' next errand will be to obtain some medical supplies from Blake in the Crimson Caravan camp. Blake can be found around the center of the camp, next to the shelves with items on them, during daylight hours. With 25 Science or Medical skill you can observe for Contreras that the supplies he's acquiring could be used to make chems, or you can point the same thing out to Blake; however, doing so to either man will have no effect on the quest whatsoever. Once you have the supplies, return to Contreras. At this point you can tell him about the chems and threaten (no karma loss) to tell the NCR and he will give you 100 caps to keep your mouth shut, after you give him the medical supplies he'll tell you he needs a "couple days" before he has another mission for you. Waiting for 24 then 18 hours seems to be sufficient to trigger the next quest.

Contreras will then ask you to deliver the medical supplies to a man named Price in Miguel's Pawn Shop. He'll also give you a passphrase: "Im looking for some betting advice" to speak to Price, and he'll tell you what passphrase Price should say to you in return. When you enter the pawn shop though, Price is nowhere to be found; instead, speak the passphrase to Keller. Keller will not know the correct response. The quest can be resolved in Contreras' favor by:
 * 1) Before saying anything else to Keller, you can exit dialogue and return to Contreras to inform him of Price's absence/Keller's ignorance of the passphrase. Regardless of which you tell him, Contreras will tell you flat-out to kill Keller and he'll take back the medical supplies (which you wouldn't otherwise be able to remove from your inventory as they're marked as a quest item). When you return to Keller, you can enter into dialogue, but your only option is to say "Goodbye." You must kill him to complete the quest. You will gain NCR infamy for doing this however. To avoid this you can attack but not kill Keller, and allow your companions to kill him.
 * 2) The Peaceful solution: With a Perception OR an Intelligence score of 7, you can inform Keller that he didn't know the correct passphrase, but this just forces Keller to expose his identity slightly earlier. He also won't be able to confiscate the medical supplies from you. With hidden speech check of 70 (ie. unlike most dialogue speech requirements this option is hidden unless your speech skill is already at required level) you can bluff your way out of conversation and close dialogue having now learned that Keller is an NCR agent. When you expose him to Contreras afterwards, he will ask you to kill Keller. You can alter his opinion (Speech 80 or Barter 70) by convincing him to make a deal with Keller, Keller can then be persuaded by either a Speech or Barter check of 80. The only other option will be to kill him.
 * 3) The other option is simply to deliver the package to Keller instead. When you do so, Keller will confiscate the supplies, expose himself as an undercover NCR ranger attempting to shut down Contreras' operation and threaten to place you under arrest. If you want access to Contreras' inventory, or you want This Machine, your only option is to kill Keller. You can do this at virtually any point in the conversation, as there will almost always be a [Draw Weapon] option during all dialogue with Keller from this point on. You are also free to exit dialogue and kill him, as he'll just stay friendly if you don't provoke him. Keller can be difficult to kill as he is an NCR citizen and killing him can incur a reputation loss with the NCR, however, it's more than possible to kill Keller before Miguel notices with a high sneak skill and a silenced weapon. Doing so and speaking to Contreras will successfully complete the quest (though Contreras' dialogue may not accurately reflect the course of action you chose and Contreras may not offer extended inventory or discount).
 * 4) Another way to complete the quest is to kill Keller. This can be done WITHOUT gaining NCR infamy by initiating combat with Keller via dialogue, then running out of the pawn shop into a nearby alley. Keller with then follow the player there. Killing him at this point without anyone seeing will result in no increase in infamy.

If you've settled things in Contreras' favor, he'll award you with This Machine and the next time his inventory restocks he'll have a wider selection of items

Siding with Keller
If you side with Keller, you will not be able to obtain This Machine from Contreras, but you will still be able to side with Boyd afterward, turn Contreras in and obtain This Machine from her. To complete the quest in Keller's favour you must have already completed Contreras' first two missions, then when he sends you to speak to Price in Miguel's Pawn Shop Keller will be there instead. The most direct way to complete the quest in Keller's favour is to give him the medical supplies.

Once you do this, he'll threaten to arrest you, you must claim you're innocent and blame Contreras. Keller will be pleased that his suspicions of Contreras have been confirmed. You then have to pass a Speech check of 60 or a Charisma check of 7 to convince him you didn't know what you were carrying. Keller will let you go free, but says Contreras will be executed. At this point you can leave, which will get Contreras arrested and you will lose access to his expanded inventory.

With a Speech check of 80, you can convince Keller to have Contreras become his informant. Or, with Barter, you can bribe Keller for 250 caps to throw his lot in with Contreras - despite the bribe, the plan remains him becoming an informant. Lacking a Barter skill of 60, the bribe attempt will fail, but you can then offer Keller 500 instead. Keller will accept and ask you to bring the deal to Contreras. Return to Contreras and inform him that Price is being held by the NCR and a Ranger has taken his place. If you were forced to give Keller the medical supplies, Contreras will demand that you kill Keller (refer to the bug where, even when killing Keller, the dialog option with Contreras is as though a deal has been cut with the ranger - the relevance of this is regarding Contreras' special stock not seeming to become available).

You could return and kill Keller, in which case consult the "Siding with Contreras" section above. If you are determined to side with Keller, then with a Speech of 80 or a Barter of 70 you can convince Contreras to cut a deal with Keller. Contreras will ask you to speak to Keller once again; you must pass a Speech or Barter check of 80 to get Keller on board; otherwise, the only dialog option left is to attack him. Convincing Keller will complete the quest, awarding you 80 XP but not This Machine; Contreras will still deal with you, but it appears that you do not gain access to his special stock.

Bugs

 * Sometimes an attempt to trade with Contreras results in the game appearing to freeze upon exiting the bartering screen.
 * Workaround: wait a few seconds and then press A, and the game should continue normally.
 * When you first speak to Contreras there will be no option to start the quest, just to browse his inventory and to leave. Possible though to hack the terminal upstairs then return to Contreras.
 * Isaac may get stuck in his house and stop appearing near the Vendortron. If this happens use something with low damage to damage him, then holster your weapon and you can talk to him.  Go look for him in his house (He lives in Isaac's House east of the Gun Runners' kiosk and next to the Hosteler Home. Entering the house will result in karma penalty, and he threatens you to leave. However, you can still initiate dialog with him after this. ).
 * After delivering the medical supplies/chems, when Contreras tells you to 'come back in a few days', you are given the dialogue option to say "So where was I supposed to meet Price, again?" even though you haven't been given that part of the quest yet.
 * Third Errand: If you pass the 70 barter check with Agent Keller before convincing Contreras to work with the Ranger, Keller will tell you to go back to Contreras and offer him the deal, but Contreras will have no more dialogue options, and the player will be locked out of completing the quest and receiving "This Machine".
 * Workaround: If you choose to just kill Keller at this point, the dialogue option to get "This Machine" should still become available. He can be killed stealthily without gaining NCR infamy.  Talking to Boyd does nothing, as there is no longer an option to rat Contreras out after making the 70 barter check deal with Keller.
 * The dialog when you give Contreras up freezes the game.
 * After the quest is completed and Contreras is locked up in one of the cells, it is possible for you to initiate dialog with him (via 3rd-person view) and browse his wares. However, the game will freeze upon exiting the buy/sell screen.
 * As well it is possible that he will not have any items in his inventory.
 * Contreras after completing the third task and obtaining "This Machine" will still sell to you, but will not give you the goods you purchase, but will gladly take your caps.
 * It is possible to not receive This Machine as a reward if you set up the deal between Contreras and Keller. If you kill Keller though, BEFORE speaking with Contreras and AFTER setting up the deal, you still get the setup dialog ("I convinced the Ranger to work with you.You're safe, but be careful") and This Machine as a reward. It seems that Keller's fate is sealed anyway, if you choose to side with Contreras. The single possibility of obtaining This Machine WITHOUT killing Keller in aforementioned situations, is to turn Contreras in to Lt. Boyd.
 * Killing Keller will at times have the dialogue option to tell Contreras that you "convinced the Ranger to work with you." Even if you immediately choose to back out of Keller's conversation, then kill him after he didn't know the passphrase. This seems to happen more often if his demise is done silently.