Q: Can witnesses receive payment for their testimony in beis din?
A: The testimony of one who takes payment to testify is invalid (Bechoros 29a). If the witness was paid to state a specific testimony, he is certainly disqualified and considered an interested party (noge’a b’davar). However, if he was paid to state whatever testimony he knows regarding the case, according to most authorities the disqualification of his testimony is a Rabbinic penalty. Thus, if the witness returned the money, his testimony becomes valid (Rema, C.M. 34:18; Pischei Teshuvah 34:24-25).
Nonetheless, if the witness incurred a loss in coming, such as travel expenses or lost wages, he is entitled to ask for compensation (see C.M. 9:4).
Furthermore, someone who undertakes to become a witness — e.g., to sign a kesubah, get or other legal document — is allowed to receive payment. Some maintain that he should be paid by both parties (see Nesivos 34:10; Pischei Choshen, Sechirus 8:45).