[epistasis]

I don't think there's any clause in US bonds that lets the US pay back the debt to avoid future coupon payments, like there is with a mortgage.

However, since there's an open market for t bills, the treasury could buy back a certain number of bonds that people are willing to sell.

However it's not quite the same. The only reason that problem don't calculate the cost of the mortgage over time is that they are more concerned with asset values, not because it's better accounting.