I'm going to get fired for doing my job, so I'm not giving them back their parts.
Why would you get fired for doing your job?
Because my manager is a complete moron.
You should talk to your boss and explain the situation because if they are a legit company they will listen to your side of the story.  If they still fire you, than you probably don't want to work for them anyway.  Maybe you should move on with your life and give them back their parts so you don't make the situation worse.
I don't want to listen to my boss, I'm going to get a lawyer to sue them.
Well if you get fired you need to return the items. They would then be able to accuse you of stealing, if they withhold your pay you are entitled to delay of pay with interest. It can be really unfair to be fired but I think a lawyer might be able to help.