I'm owed a refund - AT&T issuing credit instead

My account is Post-paid, unlimited data and I live in the U.S.

Back in the beginning of January 2018, I called AT&T to get some information about ordering the new iPhone X.  The call was long and frustrating.  I could barely understand what the AT&T reps were saying and they had a hard time understanding what I was saying so I decided to just go to my local AT&T store and get the iPhone X.  Like 3 weeks after I got my new iPhone X from the AT&T store, FedEx delivered another new iPhone X to my house (on Feb. 2nd).  So I went online to my AT&T account and I see that a new line was also added to my account and my debit card was charged like $77 for the sales tax on the iPhone X that was just delivered.  I called AT&T and they said it was sent in error and if I send it back they will refund my debit card for the $77.  I returned the phone and waited for my refund but I never got it.  Meanwhile, my bank account was overdrawn because of this charge.  I called AT&T today and was advised that - FOR WHATEVER REASON - they can't refund the money back to my debit card so I will get a credit on my next bill instead.

I'm furious!  I've been an AT&T customer for more than 20 years.  For AT&T to charge my debit card without my knowledge or authorization and then tell me that instead of refunding my money - they'll just issue a credit to my account in the next billing cycle.....it's outrageous!  I totally feel like they ripped me off.