Money is just a representational tool for value. A service rendered might make you in-debted to the person and you will have to render a service in return to get out of it. No money is involved, but if a person rendered you a big service and you return the favor with a small service, it might make the other person less inclined to help you again in a big way.
The introduction of something that represents a value is a logical step when keeping track of debt. Be it salt, cows, labor or even money.
Gift economies are of course probably hotly debated topics. I'd love to see a multi-year experiment that allocates a large area to a group and lets them try out such an economy. I don't know how they will interface with the real world to get good (medicine, electricity, ...) or if it will just throw them back into the dark ages and they'll have to progress from there.