There were more primitive systems of currency before authorities started endorsing coins. Hoards of bronze-age axeheads, few of which show signs of use, some that couldn't ever be, point to them as a means of moving wealth via standardized units. A standardized shape of metal used in trade... sounds like coinage to me.
I suspect they started out as weights for measuring out a certain value of grain or similar.
Then you stamp the weights with the mark of the sovereign that validates them.
Then at some point it becomes just as easy to just swap weights around as bring out the scales.