Send the rules. 

 1. Fear of having your idea copied/stolen is a uniquely amateur mentality. [Daniel Solis (designer)](http://danielsolisblog.blogspot.com/2013/07/what-if-someone-steals-your-idea.html) and [Gil Hova](https://gil.hova.net/category/game-design/) (publisher who references other publishers) cover the topic well in their articles about this very topic. Additionally, there is [this entry on BGG](https://www.boardgamegeek.com/thread/493249/mythbusting-game-design-and-copyright-trademarks-a). They all conclude that you should not worry about someone stealing your idea. 
 2. The company *asked* for the rules. Do you suspect they won't act in good faith? If so, the rules issue is moot. Otherwise, please know that this is standard practice. Earlier this week, I made appointments with about 20 designers for Gencon - **and declined several others** - after looking at a combination of rules and sell sheets. Let the professionals see your game for what it is!