I'm not misunderstanding, I'm correcting you. Using a country's store when not in that country isn't allowed - Apple can, and do, permanently disable accounts for doing that. If you move countries you put your new country (and payment details) on your account. If the app is in your new country's store you should (based on what other people have posted) be able to redownload it from the new country's store ; that does not apply for music, films etc, that's what your backup is for.
From the terms of use
Our Services are available for your use in your country of residence (“Home Country”).
Plus the topic is about in-app purchase from iPhone 7 with latest iOS
I've replied to the OP about that, though you're taking it off-topic. If you have a card on an account it has to be a valid card and accepted by the current iTunes validation, irrespective of whether you have a balance on the account that covers the IAP's total price.