Giving background to my question: a prominent economist was discussing the problems of the US currency and predicting it's collapse, said the following:
"...a currency is a store of value, it's a promise of a government to repay"
He continued as follows:
"People are eventually going to realise, with the US issuing even more debt, historic amounts of debt, the dollar has to go down, because we can never repay the debt...we have 250-260 trillion dollars in debt when you include all the obligations that we have in terms of social welfare entitlement programs, medicare, medicaid and it just keeps going up and up and up, and eventually the dollar is going to go down and down and down...because people are going to realise they are worthless...".
So, my question is in 2 parts as follows: As the US uses fiat currency with no physical commodity attached to it (since 1971)...
(i) How does the economist link the total national US debt of 250 trillion dollars with the US governments promise to repay when defining currency? In other words, what has 'a promise to repay' definition part of the currency got to do with the huge US debt?
(ii) The explanation of what the economist means when he defined the US currency as a "promise of a government to repay"? I mean, what is there to repay when a currency has no physical commodity attached to it other than swapping it for another dollar?
Note: The link to that programme on youtube (please fast forward to position 15:33) is here.