Ive had no experience with publishing translations of my work, so with that caveat, some thoughts.
Given Davids observation above regarding how much a book bloats when translated in Japanese, depending on the size of your book, Tom, is this guy really going to translate it out of the goodness of his heart? If he charges a premium in light of the translation, should you then be entitled to a portion of the premium?
Moreover, whats to prevent him from making additional copies of the translation and selling those extra copies? One way to prevent unauthorized copies (at least morally if not with much legal teeth) is to insist that you sign and number each copy of the Japanese translation (and the matching copy of the English-language work) and require a statement in the translation that such translation is being made with permission of the American author and that the translated copy remains the property of the American author, but so long as the Japanese owner possesses the matching set of English and Japanese version, he/she will have a license to possess and use the translated copy. The statement would also say that any copy which is not signed and numbered by the American author is an illegal copy. Will it really help? Who knows. But if youre ever in Japan and see an unauthorized copy, or an authorized without its English-language counterpart, at least you get the satisfaction of confiscating and destroying it!
Actually, this presents an interesting issue (IMHO). Can someone buy multiple copies of a copyrighted work and provide free copies of a translation of that work? There must be many nuances to this question, but Ill have to leave an intelligent discussion of these issues to those who are knowledgeable about such things.