I made a flutter application with Real Time Database of firebase which you can see your balance real time if it changes app is also updated.

You need the configure this app with your firebase app. Open your firebase console and create a flutter app use firebase CLI for configurations in your project directory ,it will create firebase_options.dart file after that create a real time database from your firebase console and don't forget to add googleservices.json to android/app directory for android. ios/runner directory for ios you need to make this while your project opened on Xcode for ios.