Kiddushin 28A

שלא ניתנה להתבע בעד אחד ⁠– מגלגלין, ממון, שניתן להתבע בעד אחד ⁠– אינו דין שמגלגלין?

where an oath cannot be imposed by one witness, as two witnesses must testify that the wife secluded herself with the man concerning whom she was warned in order for her to be obligated to take the oath of a sota, and yet one can extend her oath, is it not logical that with regard to a claim involving money, where an oath can be imposed by the testimony of one witness, that one can extend the oath?

אשכחן בודאי, ספק מנלן?

The Talmud asks: We found a source for the extension of an oath in the case of a definite claim, i.e., when the plaintiff is certain of his claim. From where do we derive that this halakha of the extension of an oath applies also to uncertain claims, when the plaintiff is not sure the defendant owes him money but merely suspects this to be the case?

תניא, רבי שמעון בן יוחאי אומר: נאמרה שבועה בחוץ, ונאמרה שבועה בפנים. מה שבועה האמורה בפנים ⁠– עשה בה ספק כודאי, אף שבועה האמורה בחוץ ⁠– עשה בה ספק כודאי.

The Talmud answers: It is taught in a baraita that R' Shimon ben Yoḥai says: The Torah states an external oath, i.e., an oath administered outside of the Temple, and it states an internal oath, an oath administered inside the Temple courtyard, i.e., the oath of a sota. Just as with regard to an oath stated in the Torah that is taken inside the Temple, the Torah rendered uncertainty like certainty, as in the case of a sota the husband’s claim is based on suspicion and yet he can extend that oath; so too, with regard to an oath stated in the Torah that is taken outside the Temple, the Torah rendered uncertainty to be like certainty, i.e., all oaths can be extended to include even uncertain claims.