Why is it necessary to give a coin to someone when you give him a present with a sharp edge or point?

Molly, an English woman, is talking to Victor, who is Asian and they are discussing different superstitions and customs.
Molly: There’s a black cat. That’s a lucky sign.
Victor: I don’t think that as it just scratched me.
Molly: Black eats are often used as good luck symbols in Europe.
Victor: You have a lot of strange beliefs. I think I was told if you give someone a present with a sharp edge or point, like a knife or a brooch, you also have to give a coin.
Molly: Yes, that’s right, then we say that the friendship will never be cut or broken.
Victor: What about crossed knives?
Molly: That’s supposed to be unlucky but we cross our fingers for good luck.
Victor: The old people of my country believe in lots of things like that but I think it’s all a lot of rubbish. I can’t see how it can affect any thing.
Molly: Well, many people say it’s bad luck to walk under a ladder and my uncle always laughed at that belief until one day he was walking under one and a pot of paint fell on his head. Now he’s changed his mind!
Victor: Do you think that number 13 is unlucky?
Molly: Not really, but airlines must as there is never a seat number 13. They go from 12 to 14 direct.
Victor: That’s only because they think that the passengers might be superstitious.
Molly: My grandmother hates to break a mirror as she says that it brings seven years bad luck.
Victor: Why 7? Why not 5 or 10?
Molly: Don’t ask me!
Victor: I have a friend who came here from Indonesia and when she first arrived she was insulted when I gave her something in my left hand.
Molly: People who travel a lot have to be very careful how they behave in foreign countries as they might do or say something that is considered all right at home but dreadful in another place.
Victor: That often happens, then people get the wrong idea about foreigners. It just needs a little understanding and common sense.
Molly: My cousin was horrified at college when an Asian student belched in front of her, then spat on the ground.
Victor: Spitting is a dirty habit but I don’t think that belching is wrong.
Molly: It’s just a matter of opinion.
Victor: That can be applied to most things in life.
Molly: You’re right.