Among the "Pesuleh Edut" – people disqualified to serve as a witness – is a "Noge’a Be’edut," somebody who has a certain bias or prejudice in the case at hand. The most obvious case of a "Noge’a Be’edut" is that of a person who received money from one of the parties to testify. For example, a lender might want to pay somebody who witnessed the loan to come to court and testify that he lent the sum in question to the defendant. In such a case, the witness may not give testimony in court. If a witness gives testimony and it is then discovered that he had been paid, his testimony is invalidated. Even if afterward he repents and returns the money, he may not return to court to give testimony, due to the Halachic rule of "Kevan She’higid Shub Eno Hozer U’magid" – once a witness gave testimony, he cannot return to testify again.
In light of this Halacha, one might, at first glance, question the common practice to pay witnesses to sign on a Get in the case of a divorce. How can a Bet Din accept witnesses who were paid for the service, if Halacha clearly disqualifies paid witnesses?
The answer lies in a basic distinction between two different kinds of witnesses. The disqualification of paid witnesses applies to giving testimony about an event that transpired. If a person witnessed an event and his testimony is needed in the court, then he bears an obligation to go to Bet Din and give testimony, without pay. For example, if a person witnessed a loan, or if, in the times when Bet Din had the authority to punish violators of Torah law, a person witnessed a violation, he must come to Bet Din to testify about what he saw. In the case of a Get, however, as in the case of the Kiddushin at a wedding, witnesses are required not to testify about something that happened, but rather as part of the ceremony. These kinds of witnesses may be paid just like anyone who provides a service. People who are brought to sign a Get may thus receive payment as compensation for their time and the inconvenience of coming to Bet Din. (Generally, either the husband and wife split the costs of the witnesses, or one party agrees to accept the expense.) Likewise, if a couple wants to bring special witnesses for the Kiddushin, they may pay them and compensate for their airfare, hotel stay and other expenses. Since their service is ceremonial in nature, and they are not brought to give testimony about something they saw, they are not disqualified as a result of their receiving payment.
Summary: Witnesses who were paid to give testimony in Bet Din about an event they witnessed – such as a loan – are disqualified, and their testimony is not valid, even if they subsequently return the money. However, when witnesses are required for a ceremonial purpose, such as to sign on a Get or to witness Kiddushin at a wedding, they may be paid for their service.