By creating the prophecy that Satan will be indefinitely evil, doesn't God inturn take away his free will?

Your partially correct. There is a sin of which is not repentable. Some call it the Unpardonable Sin. Its literal name is "Denying the Holy Ghost". There is not a whole lot of information on it, but I will try to explain it.

When you come to an understanding of God that surpasses your human existence. When your relationship with God is so developed that you have actually SEEN him and literally talked with him. When your understanding of God is based on knowledge and not faith. When there is no book or religion or person who could possibly give you any more knowledge of God that you already have...

AND THEN you rebel to fight against him...

You have commited the unpardonable sin and "denied the Holy Ghost". You are forever cast out of God's presence and shut off from all righteousness. You are miserable forever.

This is what Satan did. This is why he is who he is. Of his own free will and choice, he rebelled having had a knowledge of God. He seeks that are miserable like him.

We are therefore caught in the middle. On the one side is Satan seeking our misery and on the other side is God seeking our happiness. And we get to choose for ourselves which we will listen to. No one, not even God himself will make that decision for us.

And that, my friend, is why God and Satan are, and how the principle of Free Agency applies to both God, Satan AND us.

In the old it is written repentance leads to forgiveness but in the new it is written that you need Jesus in order for your sins to be forgiven. This is why the bible is not followed because of the contradictions of the new versus the old. The old is the true word of God the new is simply for conversion. And if we are to believe there is an evil Satan than he wouldn't have free will since free will was granted to mankind.

According to Islam, Satan was never created to be evil, he had a freewill and he used that free will to disobey God when he ordered him to bow to Adam along with the angels, he thought Adam was inferior because he was created of clay while Satan was created of fire, because of his disobedience he was destined to hell for eternity after judgment day but he challenged God that he can delude many of us and cause us to go to hell with him, God accepted the challenge but he warned him that he will never be able to delude a true believer, he can't harm any of us it's we who choose to follow him and allow him to delude us, a true believer will never be deluded ...

He already used his free will to become Satan. He can't take back his final choice any more than we will be able to once we use up our free will.