If you have read my writing titled "The Various Forms of Slavery," you know that the definition of slavery is not abolute.  While it is true that chattel slavery (people as tangible, fungible property) is the most abolute form of slavery, the definition from an economic standpoint can be much broader.  The reason for this is that as long as a capitalist can procure labor for the price of a slave, he cares not so much about the form that the slavery takes.

Analyzing this view, we first observe that the chattel slave owner does not get the slave labor for free.  Rather, the chattel slave holder must maintain the slaves.  He can't simply lock the slaves in a closet for months on end when they are not needed.  They must be fed, clothed, disciplined, trained, transported as necessary, etc.  All of these things have a cost.  The cost is that of bare subsistence, plus any labor required for oversight of the maintenance.  The capitalist must also defray the capital investment (if any) of purchasing the slave in the first place.  That too has a cost.  So, the cost of slave labor is the cost of maintenance, plus the cost of supervision, plus the cost of amortization of the capital investment.  Furthermore, since these costs must be incurred year-round whether the slave is productive or not, there is also an element of risk.  Adding all these costs together, the cost of maintaning a stable of owned slaves is comparable to the cost of bare subsistence for a free worker.

On the other hand, if the capitalist could hire workers at bare subsistence level when they are needed, and lay them off when they are not, without the responsibility to maintain them when they are not needed, the capitalist could have a distinct advantage in hiring workers rather than maintaining slaves, in terms of minimization of risk.  Furthermore, so long as the capitalist need not make any significant investment in training workers in order for them to be productive, the up-front cost of the capital invesement can be offloaded to the worker's family (i.e. the cost of bringing up a child to adulthood).

The danger of using hired help is that the price of labor could rise outside the capitalist's control through demand-side competition for labor, since other capitalists will also be bidding on this limited resource.  In fact, unless the capitalist (or the socialist administrator, for that matter) can exercise some form of control in order to keep wages at bare subsistence level, it is virtually inevitable that sooner or later, the capitalist will be forced to pay wages that represent a majority share of the labor surplus, and thus is forced to share most of his profit with the worker.  Why this is so is explained in my post titled "Capitalism versus Liberty."

So, a reasonable capitalist (or socialist administrator), might be more than willing to abandon chattel slavery if he could somehow be guaranteed a means of maintaining the price of labor at the bare minimum subsistence level of the worker.  In libertarian terms, such an artificial circumstance would require that the capitalist (or socialist or fascist administrator) initiate force to make that happen.  That initiation of force would be an act with intention to deprive people of their rightful property, and would thus constitute theft.

So, whether it's chattel slavery, or wage-slavery (contriving to rig the game so that the capitalist or the administrator has to pay nothing above subsistence level wages), or any variation in between, the essence is the same -- the wrongful intentional act made for the express purpose of depriving the individual worker of all economic benefit of his property (i.e. the value of the fruits of his labor), with the resulting profit accruing to the contriver rather than to the worker.  That is the part and parcel definition of theft.  The primary economic benefit to the antagonist is the same:  The appropriation of the value of the fruits of the worker's labor to the antagonist, and to deprive the worker of that value.  Thus, since the surplus value of the work is the thing appropriated, and it is accomplished by theft, it is fair to label any such contrivance as  economic slavery.  This is so even if the "slave" has full mobility and apparent freedom in all other respects.  Regardless of whether the economic slave is the property of another, or has full theoretical autonomy to self-determination, he still can't win, he still can't break even, and he still can't get out of the game.  In either case, or any case in between, therefore, he is still, for economic purposes, a slave.

TD