If you can, enter in a credit card from Japan temporarily so you can buy something that costs more than the balance remaining on your account and thereby use up all the remaining credit. If you cannot or do not wish to put in a credit card, go here:
and follow the instructions to report the issue to the iTunes Store. They can probably clear your remaining balance for you. Note that you will need to be physically present in the US and have a US-issued payment method before you will be able to switch to the US iTunes Store.