Case: The plaintiff sold an apartment to the defendant years ago, but only he, and not his wife (a joint owner), signed the contract. After problems transferring a mortgage, he wants to invalidate the sale with the claim that his wife did not authorize it.

Ruling: There are many sources that promote the thesis that when one who is close to the person whose authority is needed for a monetary matter acts to accomplish the matter, we can assume that he does it with their permission and authority. [We will present just a few.]
The Rama (Choshen Mishpat 96:6) cites an opinion that since women are involved in family finances, we treat a wife as an agent of her husband (even in cases where he has sole authority). Although the Rama rules not to extract money based on this opinion, the Shach (ad loc.:9) says that all agree regarding cases where one can assume she acts with her husband’s agreement that her transactions are binding. He continues that this is certainly the case when the husband knows what transpired and does not protest. The important guideline that the Shach provides is that it all depends upon the way the dayan sees it.
In this case, the dayanim view the matter as one that the wife agreed to. The sale took place and was known to her well before the argument of her alleged opposition arose. There was a previous court case, which focused on pl’s dissatisfaction with the mortgage situation, and the wife’s opposition to the sale was not mentioned. One cannot claim that the mortgage difficulties created a mekach ta’ut (a transaction based on mistaken information) because the facts were available, and if the sellers did not research the matter, they cannot void the sale.
Here there is an additional reason to validate the sale and that is the fact that the husband is a full partner in the property. The Shulchan Aruch (CM 122:9) says that one partner can represent the others in court proceedings without explicit authorization, if there is reason to believe that he represents the interests of all of them.
Even if we do not view the initial transaction as having the wife’s acquiescence, the fact that she was quiet afterwards confirms the matter (see Netivot Hamishpat 81:5). Furthermore, even if the contract was done in an invalid manner, subsequently while living in the apartment, the buyer did actions that serve as a kinyan chazaka. Since by that time, the wife certainly was aware and did not protest, the buyer’s actions should create the kinyan if still needed. Even if no one intended these actions to serve as a kinyan, still, according to many opinions, the fact that the kinyan was done within the context of a sale, where both the seller and the buyer are interested in the sale, the kinyan works according to many opinions (see K’tzot Hachoshen 275:4).