People shouldn't be leaving early to be a 3rd round pick, let alone a 6th round pick. And helping money is only really applicable when you are talking about jumping from late rounds into the top two, which he would not have done.
And I don't think you do understand my point at all if you think being a good player on a good team will make a difference in his case specifically.
He has already put together a highlight tape. He has already been a feature back. He has already rushed for over 1000 yards, caught balls out of the back field, blocked for the quarterback. His individual draft stock (disregarding the rest of the RB class) would not have been any better post-combine testing if he had stayed healthy and rushed for another 1000 this year, or come back and done it next year. He is limited by his physical measurements. A quick look back at the last ten years of NFL drafts have an extremely low number of guys with his combination of size and speed being taken in the first two rounds. Therefore, the money difference is barely relevant in his first contract as it relates to his earnings potential in the NFL.