Interesting question. Here we see opportunity for pondering contract law. Selling one's soul also suggests a definition for soul. That said one may ponder if a soldier does kill in the field of battle, although that is not legally murder by societal and legal standards, was that act of killing while under contract because the soul was not possessed by the individual?
Even though a duty to country vis-a-vis patriotic duty and/or responsibility a contract is either presumed or actually signed therefore the question of does the soul go along with it does arise. Was an autonomous and independent individual selling one's soul at that moment?
The above may be an extreme view or example, yet tailored down how does one apply that to simple contract law? An individual according to biblical reason is both not to be a lender or borrower. I am not sure of other religions so cannot comment on those points of law or tradition, yet ponder truism. The question is why not to be a borrower or lender?
If one does borrow or enter a simple contract to buy a car over a five year period we ponder your question presented. Was the soul sold for that interim period in order to fulfill the signed contract? Again, we most assuredly would need some definition for an individual's soul. Then too is that definition and the selling of the defined soul an autonomous act?
The door narrows as we soon undertake pondering the contract of employment. Are we selling our soul freely and fully to the employer only limited by the laws of man? Those would be inclusive today of contract law, labor law, and of course penal law. The employed and the employer would be limited by those laws as to how much of the sold soul could be pressed into a service fulfilled by the corporal and thinking body of the original soul's possessor would they not?
Once the or any contract is fulfilled is the soul then released and now within 'sole' position of the original soul's possessor. Or, the question asked is even though the soul is sold who says it is permanent? Who says it is full time 24/7? Where does governing these concepts come into big picture of things.
Well, once we ponder those concepts then one may ask self "when did I become the sole possessor of my soul?" Next, when did I first sell my soul and what was received as payment? How many times have I sold my soul? Did I sell only a portion or the soul in whole?