Truly no man can ransom himself, or give to God the price of his life. (Psalm 49:7)
[Lord Jesus Christ] Who gave himself as a ransomforall, the testimony to which was borne at the proper time. (1 Timothy 2:6)
While this question is posed as a possibility, and the Bible tells us that all things are possible, if we have faith and do not doubt—a very critical element when it comes to proving how genuine a person is really regarding what he or she says. This is why Mt Everest and One Tree Hill still exist and have not disappeared (maybe the tree could be uprooted and moved when a tornado passes through and a person exclaims, “Tree be moved!”—but not the hill).
I must confess that I have not transcended to such heights whereupon I do not doubt the power of God in my own life so that everything I say comes to pass; seeds of doubt affect my own aspirations, even though I believe all things are possible. For I am sure the day I cast Mt Everest into the sea, you will hear about it—right now, metaphorically speaking, I am still working on a tree on the first foothill.
Back to the question at hand. While theoretical, we are actually contemplating theodicean matters regarding the justice of God. These have to do with judgment, justice, mercy and righteousness.
Because God is righteous, He would never violate anything that demonstrated His Word was untrustworthy.
Many might think they can cite instances of God violating His own righteousness. But if they are put under the microscope with a proper understanding, those who scoff will come out the worse for wear—if you are such a one, your ignorance, if realized too late will be more than you can bear.
Being made a fool in private is one thing; in public, humiliation becomes worse when it is accompanied by disgrace and ostracism. The worst form of humiliation becomes torment. This is eternal judgment and its corollary: eternal banishment from—love, joy and peace—the presence of God.
Myself, I am nothing but a mere human who was born into sinfulness and learned how to party in the sewer of immorality—thought I was good at it too and reveled in the accolades that abounded. Yet, amazingly, someone like me is able to understand God would not violate His integrity as a Righteous Being.
Could my position today be because I understand the opposite of righteousness and deplore what being unrighteous did to me and what it can do to me still, if I were to venture once more into the iniquity pit to revel in its decadence? Or does my current position exist because I now have a better hope? A hope there is something better than being born into an evil world, governed by sin, only to suffer and then die, after having grown old with numerous regrets of not being able to experience the possibilities of true love and inner peace that escaped me because of my innate moral ineptness? (Confession is good for the soul, especially before the Only One Who Alone is righteous.)
On the point of hope, the book of Acts tells us that “God created us so that we might seek Him, in the hope that we might feel after Him and find Him (Acts 17:27).
But could God ever hope that this is might be the case for the Devil?
The Bible tells us that the Devil is a murderer and he is the very reason why sin exists. The Devil committed sin when he committed murder and, because of this, he no longer has eternal life abiding in him.
He who commits sin is of the devil; for the devil has sinned from the beginning. The reason the Son of God appeared was to destroy the works of the devil. (1 John 3:8)
You are of your father the devil, and your will is to do your father’s desires. He was a murderer from the beginning, and has nothing to do with the truth, because there is no truth in him. When he lies, he speaks according to his own nature, for he is a liar and the father of lies. (John 8:44)
Any one who hates his brother is a murderer, and you know that no murderer has eternal life abiding in him.(1 John 3:15) 
Why did the Devil commit murder? Could it be because he hated the fact Adam and Eve had been created in the image of God?
So God created man in his own image, in the image of God he created him; male and female he created them. (Genesis 1:27)
From the Bible’s account of the reason why sin exists— and because people appear to have an inherent proclivity towards hating other people at sometime in their lives for some reason or the other—one has to conclude that the above quoted Scripture is speaking truth to us.
What this means: for God to let the Devil off the hook for having introduced evil into the equation, when it was hitherto not experienced, even if the Omniscient One knew that it could exist, He would be unrighteous to do so, as justice would not be upheld.
More to the point, if God let the Devil off the hook, then others would believe they too could be evil because God’s judgments are of no avail—a scenario that sounds like the corruption found in the judicial system that currently exists in the courts of humanity.
It appears to me, evil would rule for eternity if the Omniscient Creator did not deal with the issue once for all and confine to prison—forever—all who revel in evil by transgressing the righteousness of God and violating fellow humans with their perverted and unjust selfish ways. Because of this, while mercy is a part of the deal for humans, when it comes to the judgment of God, the tables are turned on the Devil, His angels, and those who reject what our Heavenly Father has on the table for us.
Bottom line: To forgive the Devil is to acknowledge that it is permissible to practice evil. This is something a righteous God could not do. Or in popular parlance, we might say that pure wisdom would nip it in the bud and kill the thought, so that the seeds of the evil plant will not spread (Matthew 13:38).