Say I'm dealing with a subsidiary of a Mexican corporation that operates in the US. They ask for payment in MXN, but both they and I know they're going to send it to their parent corporation, converting it to MXN in the process. They and I also know that, to get the refund, they'll have to pull MXN out of their treasury and convert it back to USD. But I still did pay USD to the subsidiary—my card didn't automatically convert USD to MXN to pay them or anything.
I would expect, in this case, that my refund should happen from the subsidiary, and should be the original USD I transferred, even if that now costs them more to convert back than it was worth going in.
However, I would also expect that if the corporation entered bankruptcy, and was unwinding its debt obligations, then I, as a creditor, would probably only see the current MXN value of the parent corporation's liability to me. (Or rather, the subsidiary—who owes me the money—would unwind with no assets held to pay out, so I would then sue the parent corporation for said assets, and end up with MXN.)
That's how this works. You can call things currencies but that doesn't make it so, the legal system is an arm of he state, and states have one kind of currency, they get to decide what it is, and the they have guns and jails if you don't like it.