Answer: You replace his golf ball with a different one, causing him to lose the game in the end.
That Goldfinger is a serial cheater! After a terrible shot that causes his golf ball (a Slazenger 1) to be lost, Oddjob (played memorably by Harold Sakata) drops a new one at a favorable position while everyone is looking away. You know it's not the original, though, because you're standing on it. It's time you taught Goldfinger a lesson! At the second-to-last hole of the game, when you reach to return him his golf ball, you give him a different one (a Slazenger 7). Goldfinger does not notice the difference, and in the end you point it out to him, pretending you just noticed. According to the rules, Goldfinger loses the game, but he's not looking happy. He drops pretense and warns you not to get involved in his affairs, while his manservant, Oddjob, proceeds to demonstrate just how sharp his hat is. It seems Oddjob does not share your sense of humor, Mr. Bond!