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 dialogue 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.
Now you’re at the heart of the egg. You won't always be able to fix the problem perfectly. And you may need more time than a single phone call. But it's critical to leave the irate customer with the understanding that your goal is to resolve the problem. You may need to say, " I'm going to need to make some phone calls." If you do, give the customer an idea of when you’ll get back to him: " Later this afternoon." Or " First thing in the morning."