DISCLAIMER: This only applies to German law. I do not know the legal situation in the USA.
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You're probably not even the legitimate owner of the computer, because you agreed to transfer property on the cheaper MacBook Pro, not the more expensive one. For example, if you and I agree that I'll sell you 100 rolls of toilet paper for $10, and I deliver 150, you only become owner of 100, not of the 150, because ownership transfers through consent, not the mere act of me accidentally sending you 50 additional rolls.
You also would be liable to return the computer if BestBuy were to sue you. You gained possession (not ownership) of the more expensive MacBook Pro without a legal cause. BestBuy would be in its rights to make a civil claim of unjust enrichment against you.
Now, criminally speaking, you may be liable for fraudulent conversion. You had no intent when you picked up the computer, but now that you are fully aware of the situation and are refusing to bring back the computer, your intent is fully manifest. You desire to keep a computer that does not belong to you, and by doing so, you are depriving BestBuy of its rightful property.
There is no moral or legal doubt in this situation about what you should do.
I'm sure this isn't the answer you want to hear, but it's better you hear it here first, if BestBuy ever catches onto you. Because what will you defend yourself with? "I didn't know" definitely won't fly. And ignorance of the law was never an excuse. Bring the computer back. It's the right thing to do.