Ketubot, Chapter Thirteen, Mishnah Six
The fourth ruling stated by Admon is not connected at all to the issue of marriage, but rather has to deal with a person who signed on another persons deed to a piece of land but later claims that the land is actually his own. Note that in this and the following mishnayoth, Admon disputes with the Sages and that Rabban Gamaliel is absent. It seems that this is a second collection of Admons sayings, joined by the editors of the Mishnah to the first.
1) If a man contests [the ownership of] a field and he has signed as a witness on [its deed of sale], Admon says: He can say, [Litigation with] the second is easier for me, since the first is a more difficult person than he.
a) But the Sages say: He lost his right.
2) If [the protester] made it a boundary mark [when selling an adjacent piece of land to] another person he has lost his right [to protest].
Section one: In this case Reuven claims that a piece of land that Shimon possesses is actually his. Shimon proves that the land is his by showing a sale document on which Reuven is signed as a witness. The deed says that Shimon bought the land from Levi. Shimon says that Reuvens signature is de facto proof that he admits that the land is Shimons.
According to Admon, Reuven may claim that he signed on the deed because he preferred to claim the land from Shimon than claim it from Levi. His signature on the deed is therefore not proof that he has admitted that the land belongs to Shimon. He can still bring other proof that the land belongs to him.
The Sages disagree and state that the signature is proof of such an admission and therefore even if Reuven brings proof that the land actually belongs to him, he cannot reclaim it.
Section two: In this cases, Reuven again claims that a piece of land that Shimon possesses actually belongs to him. However, Shimon brings a deed of sale for another piece of land, in which Reuven used the piece of land under dispute as a border marker for the property being sold. Reuven would not have used this piece of land as a border marker had he thought the land belonged to him himself. Since in this case Reuven cannot say he did so because litigation with the second is easier, he has lost his right to make a claim.