There is a machlokes between R' Popa and R' Huna whether the half payment of a Tam is a kenas or mamon. The Gemara attempts to prove one side or the other by citing various Tannaic sources that seem relevant to this issue.

Later on (37b) there is a machlokes between R' Shimon ben Menasya and the Rabanan. The Rabanan hold that a person is patur if his ox gores a hekdesh ox and hekdesh is patur if a hekdesh ox gores a peson's ox. R' Shimon holds that if a hekdesh ox gores a person's ox, hekdesh is completely patur whereas if a person's ox gores a hekdesh ox the person is chayev nezek shaleim even by a Tam. The Gemara asks how R' Shimon's position makes sense: if the word "rayayhu" in the pasuk is davka then a person should be patur for his ox goring hekdesh. And if the word "rayayhu" in the pasuk is lav davka then hekdesh should be chayev for goring a person's ox. Reish Lakish answers that the starting point for a Tam is to pay nezek shaleim. When the Torah says that a Tam that gores the ox of "rayayhu" only pays half, this is a chiddush that only applies to goring a regular person's ox but not to goring a hekdesh ox. Now it is clear from here that R' Shimon holds that the half payment of a Tam is mamon and not a kenas. And seemingly the Rabanan who argue with R' Shimon hold that the half payment is a kenas. So why isn't this braisa brought on 15a either to prove that R' Popa is right that it's mamon or to show that the machlokes between R' Huna and R' Popa is in fact a machlokes Tannaim?

Tzvi Hertzberg, New York, USA