AT&T refuses to fix billing error

I placed an order with AT&T for a new line and iPad Pro on 11/19/2019 at AT&T corporate store. AT&T later informed me that iPad Pro delivery was delayed to end of December (roughly a month later than promised initially) and therefore I requested them to cancel the order and all my existing lines with them on 12/06/2018. I had a credit of $426.91 on my account at this point due to overpayment from previous month (I was going out of country for couple months). I then received a bill dated 01/04/2019 that listed a cancellation fee for existing line and and charge for an iPad Pro to the amount of 1299.99. I called on 01/23/2019 to resolve this and AT&T claimed the store didn't return the device to the delivery department, so the device charges cannot be refunded at this point. The cancellation fee was refunded as I was not under a contract and per their policy, should not have been charged a cancellation fee.

I went to the AT&T corporate store I had initially placed the order at and discovered that they did not return the iPad Pro within 7 days of order cancellation/ no pickup as per AT&T's order cancellation policy (It was roughly 50 days past cancellation date at this time). I requested them to return the device and I tracked the package. I contacted AT&T 3 days after item was delivered back to their shipping department and was told that they received the device in original, unopened packaging and my billing should be resolved on March 06, 2019. I was given a case number and post call notification email that stated this information. I called back to verify the refund on March 07, 2019 and was informed that the case has not been resolved and customer service have no ability to contact billing department responsible for refund nor am I able to speak with them. The account was showing a balance of $873.08 due at this point ($1299.99 charge added to $426.91 preexisting credit due to overpayment). I called the week after and was again informed that expected resolution date for this problem is March 06, 2019 even when I called them 2 weeks past this date (03/18/2019). But agent adjusted the balance to 0 (credit of $873.08) so my account will not go into collections.

I called again on April 01,2019 and was informed by the customer service that they cannot refund be rest of the incorrect bill ($426.91). They offered to refund $225 as a courtesy credit but AT&T will mark the account closed. I told them it is unacceptable as I am not requesting a courtesy, rather a refund for their billing mistake for a device that was never delivered to me and they are not offering the complete amount owed to me. They were not able to resolve the issue as of today (04/01/2019).

AT&T acknowledges that device was never delivered to customer (me). But is refusing to completely refund the incorrectly billed amount. Amount due to me as of 04/01/2019 $426.91. What can I do to get my money back from them?