Shevuot 46A

רישא וסיפא בראיה; ראיה דלשלם קתני, ראיה דשבועה לא קתני.

Both the first clause and the latter clause address cases in which proof is required, as even in the first clause the worker must have witnesses testifying that he was actually hired. The baraita does not mention it because it teaches only with regard to proof that requires the employer to pay, but it does not teach with regard to proof that makes it possible for the worker to take an oath and receive payment.

אמר רבי ירמיה בר אבא, שלחו ליה מבי רב לשמואל: ילמדנו רבינו, אומן אומר: ״שתים קצצת לי״, והלה אומר: ״לא קצצתי לך אלא אחת״⁠ – מי נשבע? אמר להן: בזו ישבע בעל הבית, ויפסיד אומן; קציצה ודאי מידכר דכירי אינשי.

§ R' Yirmeya bar Abba said: After Rav’s death the rabbis sent the following message to Shmuel from the study hall of Rav: Our teacher, instruct us with regard to the case where the craftsman says: You fixed two coins as my payment; and the other, the employer, says: I fixed only one coin as your payment. Who takes an oath? Shmuel told them: In that case, the employer should take an oath to support his claim and the craftsman should lose the difference. With regard to the fixing of wages, people certainly remember. The rabbis instituted the oath taken by the worker where he claims he has not been paid in a situation where it is reasonable to presume that the employer is distracted and apt to forget.

איני?! והא תני רבה בר שמואל: קצץ ⁠– המוציא מחברו עליו הראיה, ואי לא מייתי ראיה ⁠– פקע. אמאי? ישבע בעל הבית, ויפסיד אומן! אמר רב נחמן: לצדדין קתני ⁠– או מביא ראיה ויטול, או ישבע בעל הבית ויפסיד אומן.

The Talmud asks: Is that so? But doesn’t Rabba bar Shmuel teach in a baraita: If there is a dispute with regard to the sum fixed as wages, the burden of proof rests upon the claimant, i.e., the craftsman, and if he does not bring proof, his claim is dismissed. The Talmud clarifies the difficulty: Why? Have the employer take an oath, and only then should the craftsman lose the difference, in accordance with Shmuel’s ruling. Rav Naḥman said: Shmuel teaches the baraita disjunctively: Either the craftsman brings proof and receives the amount he claims, or the employer takes an oath, and the craftsman loses the difference.