Well from what I understand players get released for three reasons, skill, injury, and salary cap relief.
If a player has guaranteed money protected from any of the three then they call that guaranteed money.
For instance smith could have been guaranteed 5 mil for injury, 5 mil for skill, 5 mil for salary cap.
If he got released due to injury, he only has 5 mil guaranteed, not the supposed 15 mil.
As for paying the money back, he likely would have never received anything more than the signing bonus. And that signing bonus would be unlikely to be demanded back, but he would not have received much additional money.
We don't know the details really of the proposed contract so a lot of this is just speculation, but it's very highly unlikely he was guaranteed 15 mil for injury