Thanks everyone for your help. Let me make an additional comment about something I'm still struggling with.
It is obvious that when an employer and worker make a contract (in the realm of C in Marx's famous schema) the contract doesn't specify the concrete labor to be performed, it just says the parameters within which the boss will direct the worker. This seems like a classic case of the sale of LP. But what if someone argued that in the realm of P, we were still implicitly in the realm of exchange because for every act of concrete labor workers were asked to perform, they had the power to weigh that against the wage they were paid. Thus, the wage is really payment for specific acts of concrete labor?
