Pesachim 7A

וניבטליה בשית! כיון דאיסורא דרבנן עילויה ⁠— כדאורייתא דמיא, ולאו ברשותיה קיימא, ולא מצי מבטיל.

The Talmud asks: But let him render the leaven null and void during the 6th hour, when he burns it. The Talmud answers: Since there is a rabbinic prohibition that takes effect on the leaven, as it is prohibited to derive benefit from it after the 5th hour, its legal status is like that of leaven prohibited by Torah law, and therefore it is not in his possession and he is unable to nullify it.

דאמר רב גידל אמר רבי חייא בר יוסף אמר רב: המקדש משש שעות ולמעלה, אפילו בחיטי קורדניתא ⁠— אין חוששין לקידושין.

The Talmud continues: There is proof that the rabbis were stringent with regard to leaven prohibited by rabbinic law, as Rav Giddel said that R' Ḥiyya bar Yosef said that Rav said: With regard to a man who betroths a woman on the 14th of Nisan from the beginning of the 6th hour and onward, even if he does so with wheat from the mountains [kurdanaita], which is particularly hard and there is no certainty that it will ferment even if water falls on it, nevertheless, as it is possible that the wheat leavened, its legal status is that of leaven. Consequently, it is prohibited to derive benefit from this wheat, which is legally worthless. Therefore, if a man gives the wheat to a woman for the purpose of betrothal, one need not be concerned that it is a betrothal. The reason is that a betrothal is effective only if the man gives the woman an object worth at least a peruta. In this case the rabbis disqualify the betrothal and allow the woman to marry another man, despite the fact that by Torah law she is betrothed to the first man, as the leaven with which he betrothed her is prohibited only by rabbinic law.