redeem

This article may answer your question:

iTunes Store: How to redeem a code - Apple Support

From personal experience the iTunes store deducts the purchase price from the credit inside your account first rather than your debit card (the article makes an allusion that this is infact the case).

Hope you find this information helpful and welcome to Apple Support Communities.

It will go from your Store Creidt. Only goes from your Credit Card if your store credit can't cover the full balance, or if you bought it as a gift.