Answer

Souls are inalienable. Consequently, they cannot be sold, even to the devil. Any such putative deal would be invalid before God. It would be a mortal sin to attempt such a deal, but the deal would be invalid and would not result in the transfer of ownership of the soul. Thus the individual’s soul always remains his own, and, if he properly repents of such a deal before death, he no longer has the mortal sin either.