Is there any promise associated with money now (except "you don't go to jail if you pay enough taxes in government's money")? If not, then I can't see money as a liability, only as an investment (i.e. the government prints money, sells it for other assets (FX, bonds, stocks, etc.).
Government money is, also, a liability in a very real accounting sense too.
When the government creates money (by crediting the accounts of their providers, for instance) it is also registered as a liability for itself.
This same money always will be accepted back as a payment. That's the promise.
The widespread use of paper money is actually quite a recent innovation, compared to the millennia of history for gold and silver coinage. People didn't trust paper for a long time (and still don't, when times get troubled).
The Constitution of the United States (quite a recent document, historically speaking) invariably refers to "coining" money, not printing it.
If the value of the coin comes from the material, why issue coins in the first place?
Actually, once the state stopped recognizing gold as the standard economic unit of account, it ceases to be money. It becomes just another commodity, like corn or platinum.
Another natural consequence of gold having become a commodity is that monetary policies cease to cover the value of gold, thus it fluctuates naturally in the market just like any product, and is also subjected to economic bubbles.
The price fluctuation alone makes gold unsuited to be used as money, as it is no longer suited to store value.
Money was a liability like now. It's just that the associated promise was different. Gold was not money then more than it is now.