Earlier quoted context omitted.
If you buy a house with a mortgage, I think it's reasonable to say you 'spent' the booked value.
It's really not. If you buy a house with a mortgage, and later sell the house before the note is due, you will not owe the full amount of the accrued interest. Unfortunately the resale value of a secondhand war is very low.
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.