<human>: Can you summarize the given text in a few sentences?

I dunno if this is the best sub to ask, but /r/mechanics doesn't seem to be active enough to get a response.

I wanted to see if anyone else has ever encountered a problem like this.

Long story short, I needed to get a state inspection but my engine light was on, and you cannot get a sticker if that is the case. I take it to have it diagnosed and possibly repaired. After leaving it at the shop for a few hours I get the call and they say that my distributor gear is worn out, even though my truck runs completely fine. I ask how much, and they say it will be $325. I want a second opinion so I say I cannot afford it and ask them to get it ready for me to come pick it up. I pay the diagnosing fee, $50, and take my key and I am on my way. On the way to another mechanic that I know, I notice that my engine light is no longer illuminated. I decide to go to my usual guy that does my oil changes, and inspections, and I that I respect and trust and should have gone to in the first place. He changes my oil, everything is fine and dandy and I am on my way.

A week has gone by now, and the mechanic guy I know that recommended me to the first shop (he doesn't actually work there, he is independent and "knows a lot of people") comes by my work to tell me, "Dur oops! Guess what, those guys accidentally put in the new distributor and forgot! So, if you could just pay the remaining $275, or come back by and let them take it out, that would be great!"

I said let me think about it. I feel like he is full of shit. Has anyone else had something similar happen to them? If so, what did you do? Could I get in trouble for keeping the part and not paying, if they actually did replace it, even though I never authorized them to do so, and paid for services I agreed to and thought were rendered?
<bot>: Mechanic claimed to have left a new part in my car by accident, even though I didn't tell them to replace it, now they want me to "pay up."