I didn't change anything though, the equation remains the same. And if you move the 1 you actually move a "+1" so I see no difference in moving a "-1" as well.
A woman in a shoe shop pays for a £30 pair of shoes with a fake £50 note. The shoe salesman hands her £20. He later find that the £50 note is fake.
So he lost a pair of shoes and £20.