R. Yossi says: He (a single witness) does not arise for a transgression, but he does arise for an oath (i.e., If one denies a debt to which there is a single witness, he must take an oath against the claim of the witness.) R. Yossi says: Kal vachomer (a fortiori): If in a situation where one's mouth (i.e., admission) does not combine with a single witness for death (i.e., If a single witness testifies that a man had killed, and the man himself admits that he had killed, the two do not combine [like two witnesses] to make him liable to the death penalty) — If (in such a situation) he does swear by his own admission [i.e., If a single witness testifies that a man owes another a hundred, and he admits to fifty, he must take an oath on the rest] — then, in a situation where one's admission does combine (to make him liable) for money (i.e., If a single witness testifies that a man owes another a hundred, and he himself admits to it, his admission does combine with that of the witness to make him liable for money), then how much more so is he liable for an oath on the testimony of a single witness, (and why is a verse needed to tell us this?) — No, (the kal vachomer can be refuted, viz.:) Why is it that he swears by his own admission?
Because he pays by his own admission. Should he then swear by (the testimony of) a single witness, when he does not pay by the testimony of a single witness! The verse is needed, then, to tell us that a single witness does not arise for a transgression, but he does arise for an oath.