If you think about it there is no controversy and no clash of son vs. contract. So we have:
1. Sam is Devil's son (assuming that is true)
2. Contract sold his soul to the Devil.
However there is no indication that being Devil's son means your soul belongs to the Devil in the end. So Devil just made sure Sam's soul will belong to him, even though he is his son. Sam could turn good, do good deed, be a good Christian and so on and in the end enter heaven and his sould would be lost for Devil. Granted he is Devil's son but forgiveness is supposed to be what God is about.