Charged for the returned device

Hello all,

I was an ATT customer for a long time. Recently i upgraded two of my lines (two same devices) and since one phone had issues (random reboot) i got the phone exchanged. I returned one of the phone to ATT warehouse and they received it on 01/19/15. However i got a message from ATT that i would be charged $540 for not returning the device. I was on the phone with ATT on 02/15 and then again on 03/15 for more than 3 hours each day.

Today i was told that the issue is that the wrong IMEI device was received and so i have to pay the cost of the phone. When asked about the returned device, i was told the ATT can not do anything!!.

Just wondering what would be my recourse in this case? Obviously ATT had received my phone and they say its the wrong IMEI. This is not acceptable that ATT would keep the phone and charge the customer on top of that as well.