Bought an 8 Plus.. and it has a remaining balance from the previous owner.

I'm super (Edited per community guidelines) off. I spent the last of my money on this phone, because my fiancee had hers stolen and she needed a phone for work. When the phone arrived, i tried to unlock it through the unlock portal ONLY to be told there's a god dang balance left on the phone from the previous bum (Edited per community guidelines) owner. What do i do here, return the phone because i'm eligible or take the phone to ATT and see what can be done? I need to know soon, so any sort of help is appreciated here. Thanks everyone ahead of time.