The gemara states explicitly (Bava Metzia 3a)
תני רבי חייא מנה לי בידך
Rabbi Chiya teaches: "You owe me 100 zuz."
I showed this gemara to my chavrusa and told him that I expected to be paid in full right after shabbos. He insisted that this was only a single witness and therefore he is only obligated to swear. I argued that as a believing Jew, he cannot take an oath deliberately contradicting anything in the Gemara and if he were to take such an oath it would automatically be considered a sh'vuat shav.
Who is right?