Check receipts for cash back

I went to my local Walmart and purchased a few items, I used my debit card when I checked out. I was checking my receipt while I was walking out the door and it was for $20.00 more than the total. At the botton of the receipt-it said $20.00 cash back. I did not request or receive $20.00. When I went back to the cashier she said thats strange my drawer did not pop open, and gave me $20.00. This whole situation is still sitting strange with me-BE sure to check your receipts. I called a couple of hours later to talk to the manager, the person who answered the phone didn't know what manager was working and her walkie talkie was dead, so she put me on hold, a few minutes later she came back and said no one was responding and for me to call back in ten to fifteen minutes. I would still like to talk to some one.