Oh boy, I hate this kind of question. :-)
Good question and in 15 years of helping on this site I have yet to see a clear statement, even from the rare posts that Apple specialists make. I know I download free apps and have never had a payment method on my account, but plenty of people say Apple insists they do, even for free apps (yes, I know you want to just use Apple ID balance for your music). For example there's:
Create or use your Apple ID without a payment method - https://support.apple.com/HT204034
- "You can remove payment methods from your existing Apple ID after you sign in to the App Store, iTunes Store, or Apple Books. You won't be asked for a payment method again until you make a purchase."
Does that "until" mean yes, if you remove a card you will need to add it again when you want to make a purchase, even if you have balance or want something that's free? I don't know, but it could. Are you technically not "purchasing" something that's free, so then maybe you don't need a card once you have entered one once and removed it? You have to read Apple's documentation very, very carefully, analyzing every word. And in your case, because you are purchasing something, do you still really need a card even though you have balance?
One thing that's more clear:
If you can't remove your last payment method or use no payment method with your Apple ID - https://support.apple.com/HT203905
- " if you're using an existing Apple ID with the App Store for the first time, you must provide payment information."
Of course that's for the App Store, but maybe they mean the iTunes Store too since usually what they say for one applies to the other too?
