From the abstract:

Marx develops two different theories of the employment relationship: in one it results from a contract for the sale of a commodity, in the other from a contract establishing a social relationship. According to the first, the worker sells a commodity, which is conceived as a flow of abstract labour springing from a stock of labour power. This commodity seems to be a ‘natural’ abstraction with the properties of a productive force. Exploitation occurs when the value of labour power is lower than the value-creating capacity of abstract labour. According to the second theory, the employment relationship is based on a transaction establishing the conditions for the worker’s
subordination to the capitalist and the subsumption of his productive capacity under capital. This is an illuminating anticipation of the modern theory which considers the employment contract as an institution generating an authority relationship. It is not liable to criticisms of essentialism, hypostatization or naturalism and is able to sustain a consistent and realistic theory of exploitation, which explains it as being based on the power relationship the worker undergoes in the production process.
Now abstract labour is seen not as a productive force, but as a social relationship, and is considered an abstraction that is real in a socio-historical sense rather than in a natural sense.

Read rest here