How do I apply my Daily Apple Cash to my Apple Card balance?

Please follow the steps in the article linked below.

Make sure you have enabled the option to pay with Apple Cash first.

>>Turn payments with Apple Cash on or off

If you want to pay from only your bank account without using your Apple Cash balance:

1. Open the Wallet app on your iPhone and tap Apple Card.
2. Tap Pay.
3. Choose a payment amount and tap Pay [amount].
4. Tap your bank account, then turn Apple Cash on or off.
5. Tap the Back button, then authenticate with Face ID, Touch ID, or passcode to make a payment.<<

>>You can use your Apple Cash balance to make an immediate one-time payment. If the balance of your Apple Cash account is less than the amount of your payment, the remaining balance is paid from your default bank account.

You can choose your Apple Cash account to pay your Apple Card balance:

1. Open the Wallet app on your iPhone and tap Apple Card.
2. Tap Pay.
3. Choose a payment amount and tap the Pay [amount] button.
4. Tap your bank account, then choose Apple Cash account.
5. Tap the Back button and authenticate with Face ID, Touch ID, or your passcode to make a payment.<<

How to make Apple Card payments - Apple Support