It's rude to not give people change.
Jim is selling his TV at a yard sale for $50 and Patrick has just handed him a $100 bill.
Jim wants to make more money.
Jim offers to throw a blu-ray player in with the TV for $15 more.
Jim brings the money inside and talks to Patrick long enough for him to forget about getting his change.
Patrick takes Jim up on the offer, so Jim gives Patrick $35 back.
Patrick later realizes he's been fooled and goes to Jim's house to demand payment through threat of violence.