Case: A couple signed a divorce settlement, which stated: "The apartment will be in the woman’s possession (birshut ha’isha)." (The apartment was owned jointly.) Some time later, she (=def) sold the apartment. The former husband (=pl) protested, as he said that the clause only allowed her to live in the apartment, not to obtain full ownership of it.

Ruling: One can demonstrate from gemarot (including Pesachim 6b- see Rashi, ad loc.) and poskim that "possession" can be synonymous with ownership. On the other hand, there are sources (including Bava Kama 68b) where a clear distinction is drawn between ownership and possession. [Ed. note- Perhaps of greater significance, in Modern Hebrew, both meanings are used.]
When the text of a written agreement can be understood in different ways, the burden of proof is on the person who needs to use the document to effect a change (Shulchan Aruch, Choshen Mishpat 42:5). We assume that the people who wrote the document were careful with their language (ibid. 61:15). Therefore, def does not have the authority to sell the apartment to a third party without pl’s permission.
However, it is possible that, as a result of the agreement, the couple no longer shares practical ownership of the apartment "fifty-fifty." This is because, in addition to 50% titular rights, def has control of the apartment throughout her life. It is important to determine whether, besides living in the apartment, def can rent out the apartment to a third party and keep all of the rent. If so, an appraiser should determine what percentage of the apartment each side is to be seen to have as a result of the agreement. The apartment can then be sold according to the percentages of relative ownership.
The Rashba says that a widow, who has the right to stay in the home she lived in with her deceased husband despite inheritors’ objections, does not have the right to rent it out if she prefers. However, he explains that this is due to the widow’s lack of full rights to the apartment, and one can infer that if someone rented the home, they could rent it out to someone else. The Rambam and almost all Rishonim agree (see Tur, CM 316). Regarding Reuven who gave Shimon the right to use his apartment, the Rama (CM 316:1) says that Shimon can rent out his right to someone else. However, the Shach (ad loc. 4) cites the Maharshal who argues, and, in general, sources indicate that one should try to determine intention of the one who gave the permission. It is most likely that all agree in regard to the principle of following the intention, and they argue about what the assumption is in certain cases.
In this case, where there was an acrimonious divorce, there is no reason to assume that pl gave more rights than that which is found in the wording of the agreement. Therefore def has to prove that she had the ability to rent out the apartment to someone else. Otherwise, pl maintains 50% to the apartment, according to the way it is listed in the Tabu (Land Registry).