A man went to his lawyer and told him, "My neighbor owes me $500 and he won’t pay up. What should I do?" "Do you have any proof he owes you the money?" asked the lawyer. "Nope," replied the man. "OK, then write him a letter asking him for the $5,000 he owed you," said the lawyer. "But it's only $500," replied the man. "Precisely. That’s what he will reply and then you’ll have your proof!"
source: http://www.laughfactory.com/jokes/miscellaneous-jokes/4