You don't owe Random House squat. Having "won" you at auction, followed up by having you rewrite the book multiple times, what they did is completely shitty behavior. And it happens every day. I'm actually surprised they bothered to send you two bills. For Das Random Haus, $12,500 is pocket change and they probably cleared it from their books as a write-off years ago.
When Rupert Murdoch's NewsCorp Inc bought a company, (name deleted), they cancelled a lot of contracts... only they found other ways of saying it so CEO Jane Friedman could be quoted in the press saying, "We didn't cancel any contracts."