How can I zero my Apple ID balance?

You can either purchase an item more than the amount in your account, then pay the rest with an alternate method, or ask the iTunes staff to zero it for you.

iTunes Support - Choose your country or region - Official Apple Support

Hello ~If you are trying to zero your balance before changing regions…you may purchase an item from the App Store for more than your balance and the remainder will be billed to your credit card on file thus zeroing your balance . You may also go to upper right hand corner and click on Get Support and they can zero it for you or:

Contact Apple for support and service

Also if you are changing regions take a look through this article as well:

Change your Apple ID country or region - Apple Support

~Katana-San~