The Importance of False Promises

In order to get something, i want to make a false promise. The problem with that seems to be that you want to hold on to the idea of a promise having validity. And so when you universalize, the problem is what you universalize contradicts something else that youwant to preserve. Couldwe just look at a couple of examples here? I'm looking at a after the formula of universal law, the amulof law of nature illustrations. He uses these different illustrations to prove different things.