How honest are you?(true story happened last night)

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When i was shopping from the grocery store,the counter lady was calculating what i bought and it was around 200$ i gave her my debit card and it was showing only 20$????? she did a huge mistake and she didnt even notice anything.Honestly i told her Excuse me you made a mistake its 200 not 20.She was like oh my god,thank you very much I cant believe there exist guys like you honest and kind.I paid the right amount,held the groceries and walked away being proud of myself.Basically if someone payed her the wrong amount the poor lady will get the money dedcuted from her salary after working long hours.Honestly people what would you have done if that amount showed,Would u pay the right or false amount.Please explain with details :D