By: Rabbi Avrohom Adler
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Rav Nachman said that if a custodian swears falsely outside of Beis Din and afterwards witnesses testify against him, Rav would concede that he is still liable to pay kefel.
Rashi explains that an oath taken in Beis Din is stronger than one taken outside of Beis Din, and if one swears in Beis Din, the claim against him is dissolved.
It is also evident from the Gemora that if the plaintiff jumped up and adjured the custodian to swear before Beis Din had the chance to impose the oath upon him, and afterwards he admitted, Rav would concede that he is liable in paying the extra fifth and to bring a korban asham, but he will not be liable to pay the kefel.
The Rishonim cite Rabbeinu Chananel who explains that one who is Biblically mandated to take an oath in Beis Din, and he swears outside of Beis Din, or he swore in Beis Din before the court imposed the oath upon him, he is not exempt from his obligation and he can be mandated to swear again.
The Ramban and the Rashba disagree and hold that an oath taken outside of Beis Din is regarded as a valid oath and he would not be required to swear again. Our Gemora holds that one is not liable to pay kefel for such an oath, for it is not as strong as an oath imposed by the court.