I have traded in a good condition samsung mobile and took a s6 in November. After that due to some problem I closed the service and returned the phone within a week. I traded in the phone in Dublin,CA store and never received the old phone back. 3 months after closing the connection and clearing all the dues now I am getting a chargeback of 300$ for trade in. The store told me since I returned the new device within trial period so we will be eligible for the promo credit. But now I lost my old phone and got a bill of 300. What to do now, please help.