The term you are looking for is "legal tender". That's the thing that has the property that if you pay your debt with it, the debt is considered settled. US$ is legal tender, but ETH is not. Of course, one could try to sue for fraud and lost profits and stuff, and may even win if by some weird chance the contract specified refunds in ETH, but then good luck collecting on that judgement. More likely, contract didn't, and then tough luck.