Just going to comment on this, then I’m leaving this thread alone.
The whole point of this is that they scanned the barcode on the item that was not sold. Therefore, looking up the item that was sold isn’t going to show them anything. He’s not under suspicion for anything here - what you’re saying is only relevant if they’re looking at his receipt for some reason, which they have no reason to do unless he’s under investigation. The reality is that he’s already left the store and as far as the computer is aware, he left with the item he paid for.
As for restocking - yes, the item count will come up short next time it’s reordered. This isn’t necessarily going to set off a huge red flag though considering this type of thing happens all_the_time. It’s a somewhat big-ticket item, but I assume this store sells quite a few items in this range - I really don’t think the OP is in any danger, and if worse comes to worse, he can claim ignorance and return it for the other product when questioned. In fact, he could even feign outrage that he didn’t get the correct item in the first place. What has he done here? This is employee error, not customer theft.
I will say though that since you bring up the non-computerized system of inventory, it sounds like it’s a pretty small shop and you could kind of screw them over for keeping it.