I had an identical experience as well. About 3 years ago I was switching to verizon after tiring of AT&T's crappy network coverage. Well a couple of days after getting the phone, my work said they were going to give me a phone. So of course I went back to verizon and said 'no, dont want it anymore'. After returning everything they gave me some of the money I paid back and said the rest would be coming later. Well instead they sent me a bill for about $200 including the early termination fee. I ended up arguing with them for a bit and eventually they said they would refund it, but as they hadnt received notification of the phone being returned, they couldnt refund that. So I waited for a few more days and eventually the store did tell billing that the phone was received and I got everything credited back.
So, basically, exact same story. Theyre happy to take you as a customer, but want to give you as much trouble as possible when you leave.