I raise again the man who offers two shiny dimes in exchange for the child's grubby $20 bill. At that moment, the man would be making a good deal from the point of view of the child (if young enough).

If you introduce monetary policy into this, am I allowed to do the same?

Because of inflation, itself caused by fiat money, a monopoly on money supply and fractional reserve banking, the $20 bill depreciates over the long term while the two dimes appreciate over the long term. This is even more salient in coins that have real copper, silver or gold, of course, but it is always present when there is fiat money or fractional reserve banking and worsened significantly by the monopoly on the money supply.

So, no, the child did make a good deal. You are just not having the right calculation of time and depreciation of assets.





(Sigh, people understand this on the OC!)