It would seem many mistakenly assume that before the invention of currency economies were based upon the bartering system which is apparently not true. No economy has ever been based upon the bartering system, it has only ever been used sparingly during times of economic decline. Before the invention of currency there was what is referred to as the Gifting Economy. In a gifting economy citizens simply gave to one another without the expectation of receiving something in return. The idea is that by helping another member of society you were helping society and thus helping yourself so by giving to another you received in return whatever that person was then able to contribute to society.
Before the invention of currency a citizen was still required to pay taxes to their respective empire, the way a citizen would pay taxes is by giving their time in service of the empire. In many cases you would have servants of the empire collect those who had not yet paid their taxes and make them work in service of the emperor for a time until his debt to the empire was considered to be paid.
As I understand it historians do not agree as to why exactly currency was invented, but it is of my opinion that taxes played a major role. As populations grew a problem presented itself as to how best to keep track of who had paid their taxes and who had not. Currency solved this problem, after serving the empire a citizen would be rewarded with a certificate indicating that they had paid their dues to the emperor.
Currency also solved the problem of actually collecting citizens to make them work for the empire. The invention of currency made it so that citizens could no longer give to one another, in order to give or receive you had to have the certificate of service from the empire. Citizens did not need to be forced to work for the empire, they would come on their own because they needed to receive the certificate of service for without it they could not buy or sell.
Those who invented currency did not know that it would evolve to the degree it has today. The emperors preferred absolute power and currency was intended to make it easier for them to enforce their rule. Ultimately currency became so well ingrained into society that not even the emperor could escape the need to find ways to obtain it. Today no one is above currency, we are all imprisoned by our own invention.
Ultimately the gifting economy is the only truly sustainable economy. The gifting economy is not based upon the perceived value of certificates, but instead powered by people. In a gifting economy as long as there are people the economy is sound and people are quite the renewable resource. I believe it was the invention of currency with it's intention to empower the emperor that really increased the decline of society. With the invention of currency people began to place value upon one another, they would now refuse to help one another because they believed in the power of currency rather than the power of the person.