The reporting is frankly terrible. Let me put it in simple terms. An idea has been floating around for a while now that, in theory, you could use eminent domain to seize not just the houses, but the mortgages on the houses. Let's say that someone had borrowed $200k, but the house was now worth $100k. If you were a local government, you could seize the mortgage from the mortgage holder paying them compensation of, oh,…
The market value of a $200k mortgage for a $100k house is NOT $80k, or even $100k. Even if the mortgage is currently in default, it still represents ownership of a house worth $100k; by definition that makes it worth $100k, no $80k...
The market value of the mortgage is whatever it would actually sell for on the open market. I don't see a strong reason to believe it would always be equal to the price the underlying asset would sell for. Sometimes owning the mortgage is more valuable than the underlying property, because you've locked in a good interest rate and cash stream. Other times it's less valuable, because you have potential hassles over eviction, damage, etc., or because you've locked in a bad interest rate (the latter being the same reason bonds can be worth less than their face value).
In a situation like Richmond's housing market, with high default rates, it wouldn't be too surprising that an unencumbered property could sell for more than a mortgage on the same property would sell for. If I were buying, I would certainly demand a discount on the property to compensate for the risks of taking over the mortgage and a house with a resident in it, versus the situation of buying the property completely free and clear. That's not even specific to mortgages; any asset with a contract attached to it will be valued by taking into account the contract.