How do I put the balance back on the card

you cannot. Once you redeem the card the balance remains on your Apple ID.

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards, or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support to request a refund. When you contact Apple, please include the following information:

Your Apple ID. The refund amount. Your current shipping address.