How to gift a song from iTunes Store or Music if I already own the song

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.

See -> What you can buy with your Apple Gift Card or Apple Account balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

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• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

See also -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

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If you have already purchased a song, you cannot give that song as a gift to someone else.

Your purchased song is linked to your Apple ID account.

If you want to give the same song as a gift, you have to pay again, see -> Gift apps, music, movies, and more from Apple -> Gift apps, music, movies, and more from Apple - Apple Support

• Gifts are billed to your payment method, not to your Apple Account balance.

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If there's an issue when you send a gift

• The recipient of your gift must have an Apple ID in your country or region. For example, if you're in the United States, you can't send a gift to a friend who lives in Australia.
• In some regions, you can't send gifts. Store availability also varies by country or region. Learn what's available in your country or region.
• If you use Family Sharing and Ask To Buy is turned on for your account, you can't purchase gifts. 
• You can't gift individual songs on an iPhone or iPad. To gift a song, use your Mac or PC.
• You can't gift apps, TV shows, or movies on a Mac or PC. To gift those items, use an iPhone or iPad.
• You can't gift a free app or item. But you can tap the Share button  or the arrow button  to share a link to the app or item.