I went to the grocery store to buy some crackers. I noticed that the price was good, and I saw that they had a group of 5 boxes that were wrapped together (which I assumed came from the manufacturer that way), so I decided to buy five. When I checked out, the cashier scanned the UPC code on the bottom of one box, and assuming it was the price for all five boxes, she only charged me for one box. Apparently, the boxes were not meant to be sold in such as way, or should have been marked with a different price. Whatever the reason, I was only charged for one box, which I noticed after I checked out and looked at my receipt. The store is not owned by Jews.
My questions: Do I have a halachic obligation to pay the store back, since it was their error in not separating the boxes on display or not marking the group with a different price? Would going back and paying for the extra boxes make a kiddush Hashem, or, would the store manager think that Jews are foolish people for doing such things, since most people wouldn’t do it? Finally, if I have no halachic obligation to pay it back, could I go back to the same store and same cashier and try the same thing again, being that I’m willing to pay the full price for all 5 boxes if that is what I’m charged? Is it just the cashier’s fault and have nothing to do with me? Thank you for your attention.
As the halachah of aveidas akum and ta’us akum dictates, there is no obligation incumbent on you to pay back the money (on the contrary). However, if it will bring about a Kiddush Hashem, it is permitted, and proper, to give it back. If in doubt, do not give it back. It would not be permitted to go back and do the same thing again.
Sources: See Rambam, Laws of Theft and Lost Property 11:3; the question of lehat’oso in a lechatchilah sense is disputed among authorities, but most forbid it (rulings of Rambam 11:4, Mordechai, cited in Rema 348:2, Yam Shel Shlomo, Shulchan Aruch Harav, Onaah 11, and others).