Page 44: “Just as, therefore, in viewing the coat and linen as values, we abstract from their different use-values, so it is with the labour represented by those values: we disregard the difference between its useful forms, weaving and tailoring. As the use-values, coat and linen, are combinations of special productive activities with cloth and yarn, while the values, coat and linen, are, on the other hand, mere homogeneous congelations of undifferentiated labour, so the labour embodied in these latter values does not count by virtue of its productive relation to cloth and yarn, but only as being expenditure of human labour-power.”
My impression is that Marx is taking the same point he made before, and simply coming at it from another angle, much like a mathematician might try to prove a conclusion in several different ways. The point is still this: that by abstracting from commodities the particular characteristics to leave only value, we are also abstracting the particular sort of labor that created them, leaving only human labor in the abstract. If there is another point here, I’m missing it.
Page 45: “Coats and linen, however, are not merely values, but values of definite magnitude, and according to our assumption, the coat is worth twice as much as the ten yards of linen. Whence this difference in their values? It is owing to the fact that the linen contains only half as much labour as the coat, and consequently, that in the production of the latter, labour-power must have been expended during twice the time necessary for the production of the former.”
Note that labor-power expended is how we get labor. Intuitively obvious, but worth paying attention to. I have the ability to labor, but it doesn’t actually change anything until I exert myself. Once I have done so, that labor, if it isn’t wasted, does something. Just as an electrical current through a working incandescent bulb transforms itself into light and heat, labor-power, when expended, transforms itself into something else–namely, value.
“While, therefore, with reference to use-value, the labour contained in a commodity counts only qualitatively, with reference to value it counts only quantitatively, and must first be reduced to human labour pure and simple. In the former case, it is a question of How and What, in the latter of How much? How long a time? Since the magnitude of the value of a commodity represents only the quantity of labour embodied in it, it follows that all commodities, when taken in certain proportions, must be equal in value.”
Any commodity can be exchanged for any other because, with the correct adjustment of quantity, they can be made equal.