It's All in the Details by Colleen When Dean goes to sell his soul at the crossroads to bring Sam back, someone else steps in to negotiate the deal. Cas did get a sign and it sent him off into the past to change things. AU from The Man Who Would be King and All Hell Breaks Loose. - it's long but a WIP; latest/recent chapter in May of this year https://www.fanfiction.net/s/7002492/1/It-s-All-in-the-Details
The fixit sounds like this one
When Dean goes to sell his soul at the crossroads to bring Sam back, someone else steps in to negotiate the deal. Cas did get a sign and it sent him off into the past to change things. AU from The Man Who Would be King and All Hell Breaks Loose. - it's long but a WIP; latest/recent chapter in May of this year
https://www.fanfiction.net/s/7002492/1/It-s-All-in-the-Details