Case:


Ruling : The gemara (Bava Batra 44b) discusses the case of Reuven who sold a field to Shimon without responsibility to reimburse him if the field is taken away from him because of Reuven. Levi claims that Reuven had stolen the field. The gemara says that Reuven cannot testify that the field was his because he benefits from the field being in Shimon’s possession because his creditors can use the field for payment. If he would have had to reimburse Shimon, he would have no advantage to testify because if his creditor would take the field, he would have to likewise reimburse Shimon. Only in this case, he is influenced by the need to avoid being a borrower who does not return the money. The Ri Migash (see Ramban, ad loc.) asks how one would be more influenced by the moral need to return a loan than by the prohibition not to testify falsely. He gives two answers: 1) One who is unfit because of connection to the case (nogei’ah) is not suspected of lying but is like a relative who is unfit regardless. 2) The blemish of testifying falsely is not known to the public, as the failure to return a loan is. The answers differ as to whether there is a fear of lying regarding one with interests in a case.
The mishna (Bechorot 29a) says that the eidut of one who received money to testify is unfit (pasul). The Rishonim say that this is a rabbinic penalty for one who charged money to do a mitzva. If we say that a nogei’ah is pasul like a relative, then one who receives money but is not a party to the case is pasul only rabbinically, as the payment is external. If a nogei’ah is pasul due to fear of false eidut, this applies also to the one who was paid to testify, and the p’sul would be from the Torah. It is rabbinic only when both sides pay the witness equally. The Netivot (34:10) accepts the latter approach, making the p’sul from the Torah.
The Masat Binyamin (98) says that if one is paid only if his eidut helped win the case, it is invalid from the Torah; otherwise, it is rabbinic. The Rashba (III, 11) says that one who is not obligated to testify (i.e. the event did not yet occur) and is paid to come witness something to testify about it, it is fit. Thus, a private investigator is not pasul for receiving money to collect information, and the matter depends on if they are paid to come to court or to win.
What about the impact of professional reputation? The Ra’avad and Tur (CM 37) posit that one does not lie now because of the possibility that in new circumstances the testimony could be helpful. Thus, in case like ours that there is only a chance that helping the client will improve his reputation to his benefit, he is not pasul.