Is it permissible to demand compensation and file a complaint against a doctor after having forgiven him for the sake of Allah Almighty?

If your forgiveness of the doctor is between you and yourself, then there is no harm in suing him and demanding your right; because absolution requires a declaration, and intention alone is not sufficient. However, if you made an explicit statement of forgiveness to the doctor, then you do not have the right to sue him, because pure waivers do not require acceptance, and by waiver, the right is forfeited.