Let's give the shop the benefit of the doubt. Check the CMOS log for a change in the processor - there's your proof it was changed. Next, call the repair shop. "Hey, I had my PC in the other day, and I think yall might have put a different processor in my PC by mistake, and I need my processor back." If they say they didn't change it, kindly call them out - "Actually, the log files indicate there was a change on this day at this time, while it was in your shop." If they're smart, they'll take that as a hint that you know what you're saying and relent, or go, "Oops! Found it!!" And try to save face. If they don't relent, make their lives hell in the reviews. Make it very plain that they stole the part and lied about it even though you have proof. "Buyer beware" in big bold letters are always great buzzwords.

I hope for your sake it was an honest mistake. We're all human. But in the event that these guys are shady, best of luck to you.