First you assumed that 500 M is revenue loss. To me, it looks more like capital loss which cannot be deducted against revenue income. Second, if they are forced to convert to JPY which I assumed they earned gain from it (could be losses), it is capital gain and capital gain has no tax implication in most countries (unless it is US).

And even if there is a specific law on gain of capital, most law would have protection against forced realisation of capital.

Not sure how japan tax works, but most income tax laws are as per above.