As capital-value advanced, variable capital is the money capital advanced
to buy the commodity LP. Thus, the variable capital is but the equivalent
of the value of labor-power (and "the value of labour-value is the value of
the means of substance necessary for the maintenance of the labourer"
(Capital, vol 1, p. 171 - International Publihers). In other words, the
variable capital advanced is measured by the value of LP purchased by the
capitalist.
However, within the process of production variable capital assumes the form
of living labour (which is the use value of the commodity LP) and as such
it incorporates new value into the commodity capital (= v+s). Thus, from
the standpoint of the results of the production process, the variable
capital is measured by the amount of new value which is incorporated into
the commodity capital produced.
Eduardo