Ketubot, Chapter Eight, Mishnah Two
In this mishnah Rabbi Shimon disagrees with the opinion in the previous mishnah that states that under certain circumstances a woman cannot sell property, and that if she does, the sale is void. As we shall see, Rabbi Shimon qualifies that statement.
1) Rabbi Shimon distinguishes between one kind of property and another:
a) Property that is known to the husband [the wife] may not sell, and if she has sold it or given it away her act is void;
b) [Property] which is unknown to the husband she may not sell, but if she has sold it or given it away her act is legally valid.
According to Rabbi Shimon, if the husband knew about the property that the woman had received, she may not sell the property and if she does, the sale is invalid. The reason for this is that if this was property she had received before the marriage, and her husband knew about it, he might claim that he married her because of that money. As much as this does not sound like a good reason to get married, it certainly was a common motivation in those days (and in ours as well) and is a valid complaint of the husbands. However, if the husband did not know about the property and then she sold it, while she should not have done so, the sale is still valid. In this case, since he didnt even know about the money, he cant claim that he married her because of it.