I understand the question to be whether you can liquidate a 2% 20-year loan paying fifty cents on the dollar or whatever the fair amount is. (I guess the answer is no for the usual US mortgages but it could be yes for some kind of loans.)

I had a similar thought.

It's equivalent to buying the bond that represents the debt you owe.

Larger debt ends up in negotiations over the amount owed. War debt after WW1 resulted in greatly reduced repayments based on the original lending.

If you are a default risk on debt that has weak collateral, you could probably come to some agreement that pays back some fraction of the principal.

The US Treasuries, like 10Y and 30Y, trade on the basis like you describe.

All you gotta do is put a dollar value on it, which is actually relatively easy. Just calculate the approximate worth of 2033 dollars vs 2023 dollars, and adjust the prices today to match your expectations.

There "is no risk" in US Treasuries because we're screwed if there are risks. (Aka, everyone ignores things like the Debt Ceiling debate).

Sure, but for many reasons (including taxes and other administrative issues) having two more-or-less-offsetting positions in completely different contexts is not the same as paying a debt.