Yes, you can check the remaining balance of a device purchased with installment amount on My au (Web).
Please see below for detailed instructions.
Tap on "Device Information" [機種情報].
Tap on "Check Installment Contract and Lump-Sum Settlement" [分割支払契約の確認および一括清算].
If you have multiple phone numbers or contracts, please select the phone number or contract with an installment contract and tap on "Select"[選択].
If the amount appears in the "Remaining Balance" [支払残額] field, the operation is now completed.
Has your problem been solved?