Selling your future output net of cost-of-living is essentially what slavery is, after all; and the argument the author is making -- that if one could buy someone's future output (or the future output of a group of people) then one would have an incentive to invest in education and health care and other services which would maximize that output -- is one which dates back to at least the Roman republic, and probably earlier: Roman slaveowners would often pay to have their slaves educated or trained, because it increased the slaves' value more than the education or training cost.