Charging items me as a customer are not taking

I was charged for an item when doing groceries, the cashier needed to scan it to check if it qualifies for a discount. It did not qualify for discount, so I asked him to please leave it as I am not taking it. He did remove it from the grocery list, but not on the till. which means I paid for it. I am a busy, working, single mom and are in a rush when going into the store. I only realised this when I wanted to throw the till slip away 3 days later. This is ***** and not acceptable in any way.