There's a reason that freedom is usually held to be an 'inalienable right' (one that can't be sold). Even if, theoretically, it might benefit someone to sell their freedom, its too dangerous to allow such a transaction to be considered legitimate.
That is simply untrue. We sell our freedoms all the time. I sell big chunks of mine in two-week increments when I work; I sell my freedom for other times when I go home to an apartment where there are certain limits on my behavior.