It doesn't work like that unless you agree to that. The postnup that you would be signing isn't going to split the assets now. It's only going to address how ALL marital assets are split should you divorce. Any asset purchased during the marriage, even if it is only put in one spouse's name, is subject to the postnup. Which means that if he goes out and buys that mansion and just puts it in his name while you are married to him, he will have to give you half the equity in the mansion should you divorce. And if he doesn't agree to that, then don't sign it. And definitely take any postnup to a lawyer before even considering signing it.