The only unforgiveable sin is blasphemy against the Holy Spirit - that is, rejecting the call of the Holy Spirit to come to Christ. If you repent, not just of the sin of selling your soul to Satan but any sin, you aren't committing blasphemy against the Spirit.
As mentioned above, Jesus paid the price for your soul with His blood. Thus he has a prior claim, and the contract, if challenged, has no validity and Satan is SOL. Sux to be him.
IANAL.
I once tried to write a short story about a guy selling his soul to Satan, in return for eternal life, youth and riches. Satan agrees, the guy signs a document repudiating his baptism with the idea that Satan would never get him because he would never die.
Satan smiles and immediately drags the guy to Hell. The guy is yelling in protest, that he never got his eternal youth and life. And Satan laughs at him, and says, "I lied!'
You don't need to worry about these things. Jesus has it covered, and He has thought of everything. He is good at that.
Regards,
Shodan