You could try selling them.
Or, from : View your store credit balance - Apple Support
You can't redeem or return App Store & iTunes Gift Cards, Apple Music Gift Cards, or unused store credit balances for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can request a refund by contacting Apple Support, selecting a support category and topic, and including the following information in your request detail:
- Your Apple ID.
- The store credit refund amount.
- Your current shipping address.
Additional information might be requested for verification purposes.