Dealing with Contreras

Dealing with Contreras is an unmarked quest

Detailed walkthrough
The quest is started by either speaking with Lieutenant Boyd in the Camp McCarran terminal building or with Sergeant Contreras in the Camp McCarran supply shack.

Siding with Lieutenant Boyd
Lieutenant Boyd notes that guns and ammo went missing, and that she suspects Sergeant Contreras but is lacking proof. Proof can be found by hacking (requires Science 50) the "NCR Supply Database" inside the supply shack and downloading the Camp McCarran weapons manifest from it.

The supply shack is located in the back of the terminal building via the Camp McCarran concourse; enter the terminal and go up the escalators then 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.

Returning the manifest to Lieutenant Boyd completes the quest. She rewards the player character with This Machine and NCR fame.

Siding with Sergeant Contreras
Starting the quest by speaking with Sergeant Contreras in the Camp McCarran supply shack requires the Courier to be at least level twelve.

Asking Contreras about his supplies gives the options to pass a Speech or a Barter check of 50. Alternatively, he can be bribed with 50 caps. Contreras will then ask the player character to speak to Isaac outside of Gun Runners.

Isaac can be found either near the reloading bench to the right of the Vendortron at the Gun Runners storefront, or in his house located directly to the east of Gun Runners, behind the Hostetler home. He is willing to increase his shipments for Contreras, if the Courier passes either a Speeck or Barter check of 80, or if they give him 100 pieces of junk metal, which includes tin cans, hot plates, pots, metal cooking pots, metal cooking pans, and scrap metal. After convincing Isaac to increase his shipments, return to Contreras.

Contreras' next errand for the Courier will be to obtain medical supplies from Blake at the Crimson Caravan Company. With 25 Science or Medical skill, it can be determined that the medical supplies can be used to create chems. This can be told to either Contreras or Blake for not effect, or to Leutenant Boyd, which results in Contreras ending up in Camp McCarran's holding cells. Once the supplies have been acquired, return to Contreras. At this point one can tell him about the chems and threaten (no Karma loss) to tell the NCR, resulting in him giving the Courier 100 caps to keep silent, after giving him the medical supplies he will say that he needs a "couple days" before he has another mission. Waiting for 26 hours is sufficient to trigger the next quest.

Contreras then asks the Courier to deliver the medical supplies to a man named Price in Miguel's Pawn Shop, and provides them with a pass phrase. Upon entering the pawn shop, Price is nowhere to be found; instead, speak the pass phrase to Keller, who does not know the correct response. At this point, there are a few options:
 * 1) End the conversation immediately with "Um, well... Nevermind." Return to Contreras to inform him of Price's absence and Keller's ignorance of the pass phrase. Regardless of which he is told, Contreras will take back the medical supplies and request the Courier to kill Keller. Passing a Barter check of 50 yields an upfront payment of 150 caps for the killing. Once Keller is killed, the quest is completed and Contreras grants the Courier access to his special stock and gives them This Machine.
 * 2) Inform Keller that he has given the wrong response. With a Perception AND an Intelligence score of 7 (which always shows as fails even if successful - see Bug section), Keller informs the Courier that Price is in custody and attempts to place them under arrest as well for being involved. With a hidden Speech check of 70 they can bluff their way out via dialogue.
 * 3) Kill Keller before returning to Contreras. Choosing this option will not remove the medical supplies from the Courier's inventory, and Contreras cannot be bartered with for extra caps.

Siding with Keller
Another option is to side with Keller, by giving him the package, after which he exposes himself as an undercover NCR ranger attempting to shut down Contreras' operation and threatens to place the Courier under arrest. Passing a Speech check of 60 or a Charisma check of 7 will convince Keller that the Courier is innocent and he will let them go, which in turn leads to Contreras getting arrested.

By passing a Speech check of 80, Keller can also be convinced to have Contreras become his informant. Alternatively, Keller can also be bribed by passing a Barter check of 70 and paying 250 caps (500 caps if failing the Barter check) to him. One can then return to Contreras and convince him to cut a deal with Keller, by passing either a Speech check of 80 or a Barter check of 70. Afterwards return to Keller and pass either a Speech or Barter check of 80 with him to get him onboard.

Convincing Keller completes the quest, and awards 80 XP but not This Machine. Contreras will still deal with the Courier, without granting them access to his special stock.

Bugs

 * There may be no dialogue option to initiate this quest when speaking with Contreras. It can still be completed in Lieutenant Boyd's favor, by downloading the weapons manifest from the terminal.
 * Completing this quest may cause Contreras to stop updating his stock.
 * It is possible to obtain two medical supplies, by buying one from Blake and convincing him to give them to the player. This results in not having the option to progress the quest while speaking with Contreras, as the condition for it to appear requires the player to have only one item in their inventory. This can be fixed by using one of the supplies from the inventory.
 * After delivering the medical supplies to Contreras and being told to come back in a few days, it may be possible to immediately ask him about the location for meeting with Price.
 * After completing the quest in Contreras' favor, buying from him may result in loosing caps without receiving the purchased items.
 * After Keller replies to the passphrase (or the player character giving the incorrect passphrase), there is a 7 Perception check to allow the one to determine that Keller is a Ranger in disguise. However, the speech option is set to check the Perception of Keller, not the player character. Since Keller's Perception is below 7, the dialogue choice will never be successful. The option is also mistakenly tagged "[PER/INT]", even though Intelligence is not checked for any part of the challenge.
 * On PC, this can be fixed by opening the console, clicking on Keller, and entering the command . On all platforms, it is possible to reverse-pickpocket gear on Keller that increases Perception, such as Dr. Klein's scrubs, Dr. Mobius' scrubs, or the Advanced Riot Gear Helmet.
 * If one passes the 70 Barter or 80 Speech check with Keller before convincing Contreras to work with him, Keller will tell them to go back to Contreras and offer him the deal, but Contreras will have no more dialogue options, and one will be locked out of completing the quest and receiving This Machine.
 * On PC, this can be fixed by using the console command.
 * If Keller is killed by reverse-pickpocketing a grenade into his inventory or by a companion, NCR troopers may attack the player upon returning to Camp McCarran. Walking from a different location to Camp McCarran can fix the hostility.
 * After killing Keller, it may be possible to tell Contreras that Keller was convinced to work with him. While the dialogue is incorrect, choosing it still completes the quest, and awards the player Contreras' improved inventory and This Machine.
 * When telling Contreras about Keller not knowing the pass phrase, the medical supplies may remain in the player's inventory, while the quests continues normally.{Verify|verified}}