Change your Apple ID country or region - Apple Support - Apple Support - https://support.apple.com/HT204447
Option #1:
The above link used to have this information but now no longer exists in this form. - "If your store credit is less than the cost of a single item on the iTunes Store, App Store, or iBooks Store, you can add a credit card to your account [https://support.apple.com/HT201266]. When you buy an item, you will use the remaining store credit and charge the remainder of the purchase to your credit card."
Option #2:
"If you don't have a credit card, contact iTunes Store support." Apple will remove the balance and keep it.
iTunes Customer Service Contact - https://www.apple.com/emea/support/itunes/contact.html