If you bought something then you have to pay, if you don't pay then you owe that amount to Apple until you pay.
You can have a credit card / debit card on file and an Apple Account balance.
If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.
Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.
You can download free apps, but you may need to have a valid payment method on file, even if the app you want to download is free.
Free apps has also In-app pruchase.
See -> Buy additional app features with in-app purchases and subscriptions
Buy additional app features with in-app purchases and subscriptions
See also -> Apple Media Services Terms and Conditions
Legal - Apple Media Services - Apple
See -> If you can’t download or update apps on your iPhone or iPad
If you can’t download or update apps on your iPhone or iPad – Apple Support (UK)
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Add a payment method
You may need to have a valid payment method on file, even if the app you want to download is free.
Add a payment method
If you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.