I don't agree that money as a commodity is restricted to Volume 1. You might not have meant it exactly like this, but it seems implied in your comment that Marx only considered money a commodity in a rudimentary stage of analysis, which at a later stage is completely done away with. Certainly in Volume 3, Marx regards money in its fully developed aspects, as a pole fully separated from the commodity; but this is only justified after having traced money from a mere commodity to something more.

In Theories of Surplus Value, Marx repeats the money-as-a-commodity point in several places. I don't think he intended on restricting it to some primitive analysis in Volume 1.

If money is at first a commodity, and if over time a symbol of money representing the material predominates as money in practice, what is actually money then? Is it the symbol or the material it references? I find it impossible to understand a symbol without its referent. As much as paper money lives in an alienated sphere all on its own... I think it must, in the final analysis, ultimately point back to value. The "IOU" you mention is exactly that, a promise that there is a commodity (a value) underpinning that piece of paper, if push comes to shove. I, of course, agree that the symbol does not itself have value.

The historical research about debt is quite fascinating. Hudson and Graeber have done a great service in bringing attention to this. The fact that money is far older than capitalism is something that should make any Marxist scratch their head about the way Marx explains the historical development of money. I recall Radhika Desai, in one of the Geopolitical Economy Hours, saying something like (paraphrasing): "Money has existed for millennia, but capitalist money is of course a new development. The need for something that satisfies the various functions of money, laid out by Marx, causes changes to this pre-existing money so that it can perform these functions. Capital warps and twists this ancient category and converts it into a capitalist one."

In any case I'm happy to read your response and appreciate your insight as always. I'm always willing to learn and I don't pretend to be an expert on this.