The gemara says that when a nazir would make a shavua not to eat wine sediments, or a regular person would make a shavua not to eat neveila u'treifa, there is a possibility that they are specifically trying to swear on less than a kezayis. The rationale is that since a full kezayis is already forbidden to them, we can assume that they are swearing off something that is permited i.e. less than a kezayis. Tosafos both on 22b and 23b raises the question - less than a kezayis is not considered an issur according to Reish Lakish in Yoma 73b who says that chatzi shiur is permitted according to the Torah. But, according to R. Yochanan who says חצי שיעור אסור מן התורה, how can we reconcile him with the gemara that considers chatzi shiur to be "he'teira" and assumes that the oath would be binding. The Ritva writes that the sugya is only consistent with the opinion of reish lakish, but according to R. Yochanan who holds that chatzi shiur is assur, the oath would in fact not be binding. However, Tosafos holds that the sugya is consistent with R. Yochanan because chatzi shiur is considered איסורא בעלמא - just an issur and would not prevent the oath from being binding. Tosafos holds that since the issur of chatzi shiur isn't a full fledged issur, we don't say מושבע ועומד מהר סיני הוא. But, Tosafos 23b writes that if one would make an oath to eat a chatzi shiur, that oath wouldn't be binding because he is already pre-sworn at har sinai not to eat a chatzi shiur. Why when he makes a shavua to forbid a chatzi shiur to we not consider him pre-sworn since it is only an "issur b'alma", but when he makes an oath to eat a chatzi shiur we consider him pre-sworn and prevent the oath from being binding?
Tosafos holds that when one makes an oath to uphold a pre-existing issur, the oath cannot be binding if the issur is already in place. But, if the oath is going to increase the stringency of the issur, such as swearing to forbid a chatzi shiur of ne'veila, the oath would be binding since it would be accomplishing something - increasing the severity of the issur. But, when one makes an oath to be mevatel and undermine an issur, the oath is only binding if the Torah doesn't recognize the issur i.e. issur d'rabonon. But when the issur is recognized by the Torah, even though it is a weak issur, the oath cannot directly uproot an issur in the Torah.