Thank you for reaching out to us. I see that you'd like to obtain the remaining balance of your account. Unfortunately the balance information is not directly accessible via API. However, as a workaround, you can compute the remaining balance by subtracting the total Cost of the Account Performance Report from your known budget. Please keep an eye on our blog for future updates and announcements related to this. Let me know if you have further questions.
Thank you,
Bryan, Google Ads API Team