You can only pay yours debts with the government with money created by the government. And this is not something new.
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.
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.).