How to check my iTunes Gift card Balance?
I need to know the balance that is left on my iTunes gift card
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As far as I know, the value of an iTunes Gift Card is transferred in whole to the Apple ID account, never partially. So once redeemed, the remaining value of the gift card would be zero. How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support
The current balance (like store credit) from the transferred value of the gift card, on the Apple ID account, can be seen as described here: Check your Apple Account balance - Apple Support