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You can verify eligibility for trade-in - and an estimated value - here:

Apple Trade In - Apple

You should note that trade-in values are substantially less than the original retail cost. As such, have no expectation of recovering your original outlay.

If you trade-in a device against the purchase price of a new device to reduce the purchase price that you pay - and you subsequently return the new item for refund - the refund will match both the value paid and the payment method used.

As such, if you paid the balance by credit card, you will receive a refund directly to the credit card of the value that you paid - and may receive an Apple Gift Card for the value of the item that you traded-in.

You will not receive a payment that exceeds what you paid; you cannot “play the system” to increase the value of the refund.