A and B are disputing some fact. A wants to propose a wager, but B is too poor to pay anything to A even if A is right. So A proposes the following: If B is right, A will pay B $100; if A is right, A will pay B $50. The two look up the fact and confirm that B is right.
But A refuses to pay, claiming that if he pays $100 he will necessarily be paying $50, as he would if he were right. But he is not right, so he needn’t pay $50; and if he needn’t pay $50, a fortiori he needn’t pay $100.