Neil's mistake

I called Neil at about 20:30 yesterday just to tell him he calculated it all wrong. Not long after, he called back admitting that he was wrong and will return the £20 in the morning. I do agree with what Mom told me though. From now on, I should keep a record of how much I actually have to owe him. If he charges more, then I'll refuse to pay it. So, I'll just keep a record weekly.