Paid less for an item at REWE.

so I usually buy my groceries from Rewe because it’s near my apartment, I buy a type of onions every other day and normally I don’t check the receipt but now since I use the app I get the receipt in app and after checking it I noticed the price for onion is way cheaper (actual price is 3.99 per kg I paid 1.39 per kg) so after checking the other receipts I saw the same issue ( other type of onion was registered) so when I told the employee about the issue she first thought I paid more not less but then I told her I paid less then she said that’s good so what’s the issue. I told her I’m here to pay the difference then she just laughed and walked away.