We use debts we own to settle debts we owe based entirely on our natural ability to know who owes us a favour and who we owe a favour. Write them down and exchange them and suddenly you have a money system. Get one trusted guy to issue his iou in exchange for the one you have and you have a banker doing discounting and a standard unit of exchange in an area.
It makes sense that relative value is established first by barter, but it is not such a stretch to think that people could work the same out relatively quickly, once the money value of one common thing is established.
Also, i think i once read that at some point Japan used a system based on a standardized rice bowl.
So in earlier days it may well have been easier to set the price of a service (or anything that involved laborers) based on the number of meals needed.
He also seems to think that this is the foundation of all economic thought and thus he suggests that all of economics is wrong because they have assumed an incorrect origin of money. (I exaggerate, but not by much).