> It's an approximation in the sense that, when you buy something for 20 bucks, it's not that the something is actually worth 20 bucks, but that you value 20 bucks (and, by extension, the other things you could buy with them) than the something.

I regret to inform you that this is actually a description of abstraction, not approximation. :) The twenty "dollars" abstract over the notion of "a quantity that is owed" such that you can exchange the otherwise meaningless scraps of paper (or just bytes these days!) for other goods/services. If you were not going to use that money for something else, it actually has no value, curiously enough.

Decades ago, money had a concrete value in that you could request the issuing government to take your dollars and give you gold at an established exchange rate. When we abandoned the gold standard (and all forms of fiscal concretization), money became inherently abstract.

> Turns out money is just a more verifiable and simplified form of favors.

I should like to refer you to the book "Debt: The First 5,000 Years" for an alternative look at monetary systems, credit and debt, favors, and more. I found it a fascinating read (I mean that genuinely), and very educational with regard to this claim in particular.