Yerushalmi Ketubot 13:6 ירושלמי כתובות י״ג:ו׳

MISHNAH: If somebody protests about a field when he himself had signed as a witness⁹⁶A goes to court to claim ownership of a parcel to which B holds title, and the contract by which B holds the land was signed by A as a witness to the sale., Admon says: “The second one fits me, the first one was harder”⁹⁷Admon says that a person can argue in court that he put his signature on a fraudulent contract without declaring himself a criminal by explaining what he did. In the case in question he argues that the seller was a violent person who would have injured him had he claimed his property from him but that the buyer was a civilized person whom he could sue in court; therefore, it was in A’s interest to sign the fraudulent document.; but the Sages say, he lost his right [of appeal]⁹⁸By testifying in writing that the transaction was legitimate, he is prevented from changing his testimony. This is a general principle, “after he spoke (as a witness, cross-examined in court), he cannot come back and speak again (in the same case)”, cf. Halakhah 2:3, Notes 52–53. One has to assume here that the witness’s signature had been confirmed in court.. If he made [the field] a marker for another, he lost his right⁹⁹If A himself executed a contract in which he described a parcel as being “bounded by B’s field”, he cannot later claim that the field was not B’s. In this case, Admon will agree with the Sages..

HALAKHAH: “If somebody protests about a field,” etc. Not only if he himself made it a marker for another [field], but even if a third person made it for a fourth, when he signed as a witness, he lost his right¹⁰⁰C wrote a sales contract in which he sold a parcel to D and describing the adjacent properties mentioned the field in dispute as B’s field. If A signs as a witness on that contract, he cannot claim title to B’s field since, as noted by the Babli, 109a, witnesses never sign a document without reading it first..