One of the Pesuleh Edut – categories of people disqualified from serving as a witness – is "Noge’a," which means a person who has a vested interest in the testimony. If a person has a vested interest, then he has a personal bias and thus cannot be assumed to be objective. As such, he may not serve as a witness. It is the judges’ responsibility to determine whether a prospective witness has a bias that renders him unqualified. It might happen that some kind of personal interest can be identified, but the judges will determine that it is too indirect or not significant enough to affect the witness’ objectivity. On other occasions, the judges will determine that indeed a prospective witness cannot be presumed to testify objectively due to a certain vested interest that he has in the case at hand. This Halacha appears in the glosses of the Rama (Rabbi Moshe Isserles of Cracow, 1525-1572) to the Hoshen Mishpat section of the Shulhan Aruch (37:18).
The disqualification of "Noge’a" also applies to a witness who is paid to testify. If a person happened to witness an incident, and a litigant wishes to hire him to come to Bet Din to testify to what happened, he is disqualified. Since he is on the litigant’s payroll, so-to-speak, he necessarily comes to the courtroom with a predisposed bias in favor of that litigant.
In light of this Halacha, the Poskim address the question of whether the Rav Ha’mesader – the Rabbi officiating at a wedding, who is customarily paid for the service – can serve as a witness on the Ketuba or for the Kiddushin. Since he receives payment for overseeing the wedding, perhaps he should be deemed a "Noge’a" and thus disqualified from serving as a witness.
Although some authorities indeed conclude that an officiating Rabbi should not serve as a witness at a wedding, the general consensus among the Halachic authorities is that he may serve as a witness despite his receiving payment. One reason is that a witness is disqualified if he receives compensation for the trouble of coming to Bet Din to testify. The Rabbi attends the wedding irrespective of payment, and thus although it is customary to pay him a fee, he would not be considered "Noge’a." Moreover, in the case of a wedding, the witnesses’ job is simply to be present and observe; they are not there to provide information that a court will act upon. The acceptance of payment quite obviously has no effect on the witness’s function to observe the ceremony, and therefore the fee paid to the Rav Ha’mesader does not undermine his validity to serve as a witness. Hence, as noted in the work Mesos Hatan, the consensus among the Halachic authorities is that an officiating Rabbi may serve as a witness for the Ketuba or Kiddushin even though he receives payment for his services.
It should be noted that, as several sources indicate, there are three functionaries whom it is appropriate to pay for their services at a wedding – the Rabbi, Hazan, and Shamosh. Some Rabbis found an allusion to this practice in the verse "Rahash Libi Dabar Tob" ("My heart stirs with good things" – Tehillim 45:2). The word "Rahash" may be read as an acrostic for "Rav," "Hazan," "Shamosh," and the verse here speaks of affording them "Dabar Tob" – "good things" – referring to payment. It is therefore appropriate for couples to ensure that some payment is given to these three functionaries for the services they perform at the wedding. It is told that a couple once consulted with a Rabbi after being childless for several years after their wedding, and he drew their attention to the fact that they had not paid the Rabbi, Hazan and Shamosh who served at their wedding. He noted that if these three people are not paid, then the word "Rahash" is turned into an acrostic for the names "Rahel," "Hana" and "Sara" – three women in Tanach who suffered infertility for many years. This demonstrates the importance of following this custom and paying the Rabbi, Hazan and Shamosh for their services at a wedding.
Summary: A person may not serve as a witness in Bet Din if he has a vested interest in the case that undermines his objectivity, including if he was hired by a litigant to testify on his behalf. Nevertheless, a Rabbi paid to officiate at a wedding may serve as a witness for the Ketuba or the Kiddushin. It is customary and proper to pay the Rabbi, Hazan and Shamosh for their services at a wedding.
