@user5924704 thank you for your response.
The very correct way ( and assuming that this was an award of some kind from US sources and not funded by a foreign entity ) would be for you to file 1040-NR return recognizing the US sourced income, claiming Foreign Tax credit ( for taxes placed on this income by your tax-home country ( Japan).
Assuming that you were in US for less than 90 days, and if NO tax docs are issued to you by the disburser of the award, that you just recognize this income on your Japan taxes and ignore the US filing altogether --- this position/ possibility is based on the small amount of the award and the double taxation clause of the tax treaty between US and Japan.
As I said the pref. way is the first option.
