Walmart / unauthorized billing

i was in walmart shopping and only spent 40 dollars. the next day i checked my my account and found out by my bank that i had been charged 250.00 dollars and they couldnt even tell me what it was for. they did say that the charge was from walmart and when i called the store the next day, i was told by a manager, who looked up the transactions that, they had no record of the 250.00 being charged to me card. so i called the bank back and they told me that the charge was still pending and i would have to wait 7 days . even though my bank said that they would dispute it, im still mad because how can someone charge you for something and not have record of it. that makes no sense cause even pending transactions are still on records. i am no longer a walmart shopper and they are a rippoff!!!