Rambam - 1 Chapter a Day

A person who has a vested interest in the outcome of a judgment is likely to lie or at least distort the evidence (perhaps subconsciously) in favor of the desired result. The Rambam, however, goes further, stating that when a person has a vested interest, it is as if he is a principal in the case and he is disqualified as a witness by Scriptural Law. Even if there is no suspicion at all that he will lie, he is unacceptable. See the commentaries of the Sefer Me’irat Einayim 37:1 and the Siftei Cohen 37:1 where this subject is discussed.

For they have a vested interest.

As validated by a kinyan suder, the exchange of a handkerchief; see Hilchot Mechirah, ch. 5.

I.e., the fact that he was once an interested party does not disqualify him after he withdraws his interest. Although as stated in Chapter 14, Halachah 2, a witness must be acceptable both at the time he observes the matter of concern and at the time he delivers his testimony, the Rambam maintains that this principle does not apply in the present situation because the difficulty with the witness does not concern his person, but his vested interest. Hence, when he rids himself of that vested interest, he is fit to testify. Tosafot, Niddah 50a, differs and maintains that in this instance as well, if a person is not fit to testify when he observes the evidence, his testimony is not acceptable. This view is also cited in the Hagahot Maimoniot.

Bava Batra 43a, the source for this ruling, mentions only that such an individual may serve as a judge. The Rambam extends this conclusion and applies it to witnesses as well.

Since the person will continue to listen to the Torah being read from that scroll, he will still be deriving benefit from it remaining in the city.
The Radbaz states that, according to the Rambam, this ruling applies even when the city also possesses another Torah scroll. The Shulchan Aruch (Choshen Mishpat 7:12) and the Ramah (Choshen Mishpat 37:19) quote the opinion of Rabbeinu Asher who maintains that if the city has another Torah scroll, a judge or a witness may withdraw his share in the Torah scroll in question.

Questions involving other religious articles that are required for communal use [Ramah (Ibid.)].

As reflected in the continuation of the Rambam’s statements, since the poor people are dependent upon them, when the poor people receive money from other sources, that will release their burden. Hence, they are considered to have a vested interest.

And thus they would have no immediate benefit from their testimony.

For in this way, in the future, they will not have to give them the same amount. Alternatively, the general benefit to the city will also affect them in an indirect manner.

When, however, the poor are supported from donations coming from outside the city, the inhabitants of the city may testify if they take precautions so that they do not have a vested interest (Radbaz).

I.e., he claims that the person who sold the land to the partners stole it from him and sold it illegally. Moreover, he brings witnesses to prove his claim. In such an instance, he has the right to expropriate the land and the partners must sue the thief to receive reimbursement.

I.e., he would be testifying that the witnesses who testified on behalf of the claimant were not acceptable because they transgressed. Hence their testimony would not be fit to take into consideration.

I.e., even the portion of the land belonging to the other partner.

He may, however, withdraw his ownership after the protest has been raised [Shulchan Aruch (Choshen Mishpat 37:1)].

I.e., he gives his portion to him as a present without receiving anything in return. He will lose his portion of the field regardless - for his own testimony will not avail him. Nevertheless, by giving his portion to his partner and testifying, he prevents his partner from suffering a loss.

We are speaking about a creditor from whom the partner borrowed money before he divested himself of a connection to the land. That creditor thus has a lien on the land and is entitled to expropriate it from the person in possession. Accordingly, the partner would still have a vested interest in keeping it in the other partner's possession. When, however, he undertakes a commitment to make restitution if the field is expropriated, it does not matter to him at all who is in possession of the field. Hence he may testify about it.

For if it is granted to the claimant, he is entitled to all its produce. The sharecropper receives only compensation for his expenses - and that only when the yield is sufficient to pay those expenses.

Even if the claimant is given the field, the sharecropper is not obligated to pay him for the previous years. Nor do we say that the sharecropper fears that he will lose the opportunity of working this field, for we assume that there are many fields of similar value available. Hence it is not considered as if he has a vested interest (Kessef Mishneh).
This is speaking about a situation where the sharecropper did not till the field and prepare it for sowing before the claimant demanded its return. If he had tilled the field, he may not testify, because he has a vested interest since all the work he invested will be for naught if the field is expropriated (Radbaz).

The Shulchan Aruch (Choshen Mishpat 37:12) states that the owner may give the rent back to the renter to enable him to bring it to court so that he may testify without any vested interest.

Even though he is entitled to receive reimbursement from the person who rented him the field, that requires suing him in court and no one enjoys the possibility of entering legal proceedings (Siftei Cohen 37:13).

This applies even with regard to an ordinary guarantor, not necessarily an orev kablam (Radbaz, Kessef Mishneh).

If, however, Shimon does not possess a field, but does possess movable property,
Reuven is not acceptable as a · witness. For the movable property can be concealed or disposed of and the debt left to Reuven (Radbaz).

Hence even if the field is expropriated by Yehudah, Reuven will not suffer a loss. If, however, Shimon does not possess another field, Reuven is considered to have a vested interest in Shimon retaining possession. For if he does not possess the field, Reuven may be called upon to pay the debt (see Bava Batra 46b).

Reuven purchased a field from Shimon and afterwards, Levi purchased a field from Shimon. Yehudah then comes to Levi and claims that he, Yehudah, is the rightful owner of that field and it was stolen from him. Reuven can testify on behalf of Levi and establish him as the rightful owner of the field.

That is in his possession and not encumbered by any loans. In this instance, as the Rambam continues to explain, Reuven is protected, because he can always sue Shimon and expropriate the unencumbered field. If, however, Shimon does not have. such a field, Reuven is considered as having a vested interest for he has no security for his purchase as we will continue to explain.
The Ra’avad adds that Shimon must designate this field as an apotiki for Reuven, assuring him that he will receive it if the field he purchased is expropriated. The Shulchan Aruch (Choshen Mishpat 37:14), however, does not mention this requirement. See the Siftei Cohen 37:18

I.e., Dan, a creditor of Shimon’s, comes and seeks to expropriate the field Reuven purchased. Had Yehudah not prevailed against Levi, Dan would have expropriated the field Levi purchased, for that was purchased after Reuven’s field. Nevertheless, even though that field is not available, Reuven will not suffer a loss, because he can rely on the unencumbered field. Hence, it makes no difference to him whether or not the field remains in Levi’s possession.
If, however, Shimon does not possess an unencumbered field, Reuven would derive an advantage from the field remaining in Levi’s possession. For then the creditor would expropriate Levi’s field first.