Responsa על כתובות 165:6

Q. A wants to buy a house. He is afraid to do so, however, lest his wife later legally restrain him from reselling it [since a wife's ketubah constitutes a lien on the husband's immovables]. Should A stipulate that he buys the house on condition that his wife be unable to restrain him from selling it, and if she formally agrees thereto, would such an agreement be binding on the wife; or would it be null and void since she would thus have agreed to a thing that as yet had no existence?
A. The above agreement would be binding on A's wife, for a person may relinquish his claim to rights and privileges before they are created.
SOURCES: Cr. 263; Mord. Ket. 212; Hag. Maim. Ishut 23, 2. Cf. Moses Minz, Responsa 39.