The original article is here.
Marx only works through the concept of labour power when he does because, like a proper student of Hegel, he knows you cannot fully develop a concept without it being worked up to or it functions as an ungrounded assumption, therefore he can't have a completed concept of labour power in chapter 1. He also knows that you have to work from within the categories of political economy itself, that is, you have to follow the immanent logic of the arguments, not introduce your own "first principles" ex nihilo.
He has to work through the notions of value presented by rationalism, empiricism, and Absolute Idealism, so that he can lay out the limits of each concept. Abstract labor is thus not the same as labor power, though he refers to it as labor abstracted from any actual labor, that is, as potential. By the time we have gone through the critique of earlier conceptions of value (binaries of use-value vs. value, concrete labor vs. abstract labor, and then the form of value or exchange-value leading to the money form as value), all of which are partial truths, but therefore also untrue because partial, he lays out the ground of value in the fetish characteristic of the commodity.
He then has to talk about the role of exchange in the REALIZATION of potential value. Commodities are not bartered, they are an exchange of equivalents. Money is the necessary social form of exchange-value because it is only through a commodity that is the (empty) universal, the pure, immediate concept of value. Money allows exchange to take place without any direct consideration of the use-value of the commodity by the producer, and it retains value, that is, despite the change of form of value, it retains it's substance as a quant of wealth. Money is also one kind of materialization of value in exchange (just as we started with the commodity as the first materialization of value) and it is essential in a system of indirectly related values that the validity of the common substance (labor), and its magnitude (value) magnitude, and that magnitude of the substance is measured by the socially-necessary labor time.
This is why Marx spends chapter 3 discussing what money is, at this point and insofar as it has been developed. Commodities cannot circulate as commodities, and generalized commodity production therefore cannot be the general form of human social relations, if commodities are not exchanged for money, if their value is not realized in a universally recognized form independently of their use-value and of concrete labor. This is why Marx moves from money as measure of value to medium of circulation, to money proper (hoarding, means of payment, and universal money.) It is with universal money that we see money achieve it's perfected form: "it strips off the local garbs... It is only in the markets of the world that money acquires to the full extent the character of the commodity whose bodily form is also the immediate social incarnation of human labour in the abstract. Its real mode of existence in this sphere adequately corresponds to its ideal concept." It is necessary to keep in mind that there is no way for this to be Marx's completed concept of money in his work because it would have to deal with states and the world market, which we know Marx never got around to completing, but the first working up of money that allows us to conceptualize capital.
Note that when Marx gets to The General Formula for Capital, we now have a circuit, a movement of valorization in which value moves through different forms (commodity, money) which are intended to preserve (and in fact increase, but we are not there yet) the magnitude of value through the entire cycle. Capital is not the means of production or money or commodities, it is really the entire circuit in which money, labor, means of production, commodity are all nothing more than moments of capital in its extended self-reproduction.
The Value-Form people get stuck in thinking that because exchange via money is a necessary condition for the realization of a measurable magnitude of value (substance), the substance is created in the act of exchange. They conflate value as potential versus actual. Exchange does not create the value-form of social wealth, rather it validates how much of the magnitude of that substance COUNTS. If none of the commodity is sold, the commodity does not COUNT, is not validated, as value, but this has direct and dire cnsequences for the producers because the result is bankruptcy and unemployment. The substance of the commodity, abstract human labor, is already a commodity, and since it is already valued, we do not enter capitalist production until we already have spent money on labor and means of production. This becomes the ground of the bare minimum required to reproduce the conditions of production, that is, the amount of value that must be validated or realized in exchange for the process to happen again is already the potential value, independently of and prior to the exchange of the commodities produced.
Money does not equal value-form. Exchange-value is the mode of appearance of value, that is, value only becomes visible in exchange because in an indirect system of social reproduction there is no direct measure of the validity of the labor-time of this commodity versus that commodity; there is a dynamic, indirect validation of labor-time based on its social necessity (that is, based on its labor-time in relation to other labor-times.) Money is the universal, meaningless, empty, but necessary form of appearance of exchange-value expressed as price, and the price at which the commodity sells is the validation of the amount of socially necesssary time in its production. This price may be above or below what would allow for the reproduction of the amount originally spent in the first place, and we only learn out of the exchange the validity of the value, how much of it counts.
The Value-Form milieu in effect ignores the fact that the first commodity for Marx is labor-power, which as he says in chapter 6, having sold its hide, heads off for a tanning (and since now labor-power is going from the realm of exchange, where it too was a commodity exchanged for money, for the universal equivalent, it must now go into the hidden abode of production for a good tanning. To put it another way, the current value-form milieu seems to want value without labor or production, which I would suggest is a side-effect of the transformation of capitalist society in the late-20th and 21st centuries as noted in different, potentially complementary ways, by Krisis, Exit!, and Postone on one side, and Robert Meister and Randy Martin on the other. Norbert Trenkle's "Labour in the Era of Fictitious Capital" is succinct on this point, and if we consider the value-form milieu as succumbing to the financialization of capitalism and the mass expulsion of labor from the production process, we can see the root of their hypostatization of money.