I was wondering, let say we perform any wrongdoing it will be count against us but example if we go to buy 10 pcs of something priced $1 and after giving to the cashier, he miss counted as 9pcs and we pay $9 eventhough he put everything in a pack and we count as 10. Ignoring this situation of him miss counting the item and we pay less, are we wrong of not correctly pay him?

If you were the seller, do you accept that the buyer whom you gave more money, take the money?
I am sure you do not, because you will pay it from your pocket.
Hence, it is haram.
You have to pay the money back to the seller, even if you were in a non Muslim country, because you will effect the image of Islam.