As Marx's dialectical argument argument develops--and the greatest difficulty is understanding the kind of necessity which moves us from simple to expanded to general to the universal form-- the stabilization of a universal equivalent form means that commodities no longer take the equivalent form and thus manifest an aspect common to all of them--that they too are materializations of social labor time in the abstract. As a result, commodities no longer buy other commodities.
The inner contradiction of commodities--that their respective values in use are mutually exclusive of the value expressed in exchange--is reified now as an external contradiction between the relative and universal equivalent form, commodities and money.
Which means that money excludes commodities from manifesting one aspect of their contradictory essence--this ex-cluding or contradictory nature of money seems to me Marx's more important point than the reduction of money to a commodity; in other words commodities never themselves count as depositaries of abstract social labor time.
They are henceforth simply use values, concrete things, never themselves universal or immediate representations in exchange of some aliquot of abstract social labor time.
So to count as such--and counting as such is why commodities are in fact produced, in fact what they are produced as--they must metamorphosize into money, but given money's monopoly or its contradictory essence there is no reason that money's possessor will part with it therewith to purchase commodities. A sale does not mean a purchase.
Commodities can of course still make the salto mortale virtually on the basis of the purchaser's credit, but if this dries up...
Rakesh