Ok, I think this is all the tags I need.
Sam was feeling Dean would be better off with John so makes a deal with a demon to bring him back. It takes seven days and during that time Sam is invisible as are his possessions. The demon feeds of his emotional pain.
Dean feels something is wrong but doesn't remember his brother. ]
John stumbles over Sam's tote bag and finds a letter that makes him remember.
It turns out Sam had been spending his remaining time near Dean while he was working on the car by staying in the back seat.
When they realize he appears and keeps saying "It's a good deal."
The story takes place at Bobby's.