Labor (installation of alternator)

I bought a brand new alternator and brought it to these guys to get it installed. As I was waiting for the service, they kept telling me I can not look as the mechanic is installing the alternator. It sounded weird so I walked outside to see what's going on from a little far away. The mechanic came up to me and said he is feeling nervous when I'm watching. They asked me to sit in the office. I did what they said, feeling very uncomfortable. I knew something was fishy. After a while they brought me my car. The minute I turned on the car it was making a weird noise. I told the mechanic about the problem, and he said he can't do anything about it. I went to another mechanic to see what's wrong and I found out that they put a used alternator in my car and kept the brand new one that I gave them to install. What a dishonest service!!! Now when I call them, they tell me they have to charge me again for labor to install a new alternator. How about the one they stole from me?