What do you think?

Ok… here's what happened. I went in to a T-mobile store here in Seattle last Friday night. I needed a new screen protector. It was a few minutes before closing, but that didn't seem to be a problem for them. They took my phone out of the case and applied the new screen protector. There was no charge since I have the protection plan. They gave me the phone back. I didn't inspect it, assuming it was the same but with a new screen protector. I was the last customer, and the only one there besides the two employees that were both helping me. I watched one of them apply the screen protector, remarking how easy they make it look. The other employee remarked, "We do a lot of them ". They gave me the phone back and I left. 
Not until I was home did I notice they had failed to put the top of the MagSafe Otterbox case on my phone, thus leaving a gap between the phone and the rest of the case. Basically, this left me with a case that didn't function as designed. 
I couldn't go back that night because they were closed. I went back the next day and they said they had thrown it away and it was inaccessible, and that I had to come back Monday and talk to the  manager. I went back Monday, but the manager was gone for the day, so I went back Tuesday. I spoke with the manager about what happened and he told me there is nothing they can do. I don't think I should have to suffer the loss and pay for a new case. I feel like they were negligent. Who is responsible?