Something about this doesn't make sense. Either he pays in BTC or JPY. If he pays in JPY, then he has to sell the BTC, which if he does that on market, will tank the price. If he pays in BTC and the courts recognize it, then BTC must be officially recognized as a currency, no? Not sure.

In any case, if the courts recognize that he can pay back in BTC, and he has enough BTC to do so, then I don't see why it's contingent on the BTC price. He could do it today.