It seems like only yesterday I posted a nice kashe my son had on the last Mishna in Horiyos when he made a siyum on Mishnayos Nezikin, and now he is just about finished with the entire seder of gemara. In his siyum on Shavuos he chose a particularly complex sugya to speak about, so apologies if I don't capture this idea very well.
R’ Yochanan (Shavuos 37) holds that if Reuvain swears falsely that he doesn’t owe money to Shimon and two witnesses contradict him he is chayav a korban. However, if Shimon produces a shtar that says Reuvain owes the money, Reuvain does not have to bring a korban. The reason for the distinction: a shtar creates a lien on property and there is no penalty of a korban for swearing falsely with respec to property.
Tosfos (Shavuos 37b, B.M. 4b) asks: R’ Yochanan holds “shibuda d’oraysa” – every loan automatically creates a lien on property -- so how can anyone ever be chayav a korban shevua? Any dispute about a loan is essentially a dispute about property!
Tosfos answers that the case of korban must be where the person who made the loan forgives and is mochel this shibud.
Tosfos now goes a step further: the Chachamim made a takanah that says if Levi buys Reuvain's property unaware that Shimon already has a lien on it, if the agreement between Reuvain and Shimon was only an oral contract (milveh al peh), Shimon cannot collect the land bought by Levi; there is an automatic mechila of the lien on the property. This protects Levi (lekuchos) from having his land snatched because of circumstances he has unaware of. However, if a shtar was written, Shimon can take the land from Levi because Levi should be aware of a documented loan and lien against Reuvain's property that he purchased.
Based on this takanah, Tosfos works out that the case R’ Yochanan must be speaking of is where Reuvain sold land which had a lien on to Levi, and Reuvain has no other personal property left. Thus, R' Yochanan said: If there are witnesses but no shtar to the agreement between Reuvain and Shimon (i.e. it's a milveh al peh), Shimon can only collect directly from Reuvain (because the takanah does not allow him to collect from Levi), and Reuvain has no land in his possession. Since there is no property under dispute, Reuvain is chayav a korban if he lies under oath. However, if there is a shtar, Shimon can now try to collect the land which Levi bought from Reuvain. Since this dispute now involves property, Reuvain is not chayan a korban for lying under oath.
The Ran goes through the same basic question and answer as Tosfos, but omits the detail that Reuvain personally has no land. My son suggested that this omission is significant and reflects a fundamental disagreement between Tosfos and the Ran as to how this takanah which prevents Shimon from collecting from Levi in a case of milveh al peh works. R' Shach in hi Avi Ezri Ezri is choker whether the takanah means: 1) The Chachamim said a milveh al peh cannot create a lien or shibud; or 2) A milveh al peh creates a shibud, but the Chachamim negated the power to collect on it.
If the Chachamim said that milveh al peh cannot create a lien, then even if Reuvain himself owned land, Shimon could not collect it. However, if milveh al peh does create a lien, just the Chachamim blocked the ability to collect on that lien in order to protect lekuchos, that would not protect Reuvain himself. My son suggested that Tosfos takes the latter position and therefore writes that we are speaking of a case where Reuvain has no land; the Ran takes the former position and therefore has no need to include this detail.
There is a part II and even a part III to this that my son did not say over, but ad kan hakafah aleph.