In Texas Express, the only ball that spots is the money ball. In tournaments, this is only the nine. In a money game, it is also whatever ball the spot is.
The way I have always played it is this-
Say I'm spotting you the 6- If I make the 6 on an illegal hit, or on a push, it gets spotted back up. If you make it on an illegal hit, it stays down.
It's just not fair to spot someone a certain ball, and then be able to get rid of it illegally. Now, if he accidentally pockets it on a foul, too bad. He just made the game tougher for himself. It stays down then, because it would also be unfair for me to have to shoot another ball because he fouled.
Look at it another way- If I can make the 6 on a push, and you say it stays down, even though that is one of your two money balls, then you can pocket the nine on a push. Now you have one money ball- the six, and I have none. I can't win the game no matter what.