Hi i am actually not sure If i already sent in a similar question so I apologize in advance .
The gemara indicates in daf 55b that where the rabanon put a stolen animal in ones posession he can be chayav kares even doraissa. I believe there is a similar concept by mezikin (bor) and chometz.
So why elsewhere does the gemara seem to indicate that hefker bd hefker will only effect a kinyan drabonon?
Moshe rubin, Brooklyn, ny
1) This question is asked by the Maharitz Chayos on 55b. He cites the Machaneh Efrayim (Hilchos Mechirah, Kinyan Meshichah 2) who writes that Chazal are able only to make Hefker, but not to make a Kinyan. How, then, can Chazal take the stolen item away from the real owners?
The Maharitz Chayos answers that here, in addition to the Hefker of Chazal, there is also the despair, or the "Ye'ush," of the owners. Therefore, when the thief performs the Kinyan of putting it in his yard he effects a Kinyan Chatzer, or if he pulls it he effects a Kinyan Meshichah.
In other words, although Chazal can only create hefker, in our case this is sufficient to combine with the Ye'ush of the owners to create a Kinyan d'Oraisa.
2) The Gemara in Pesachim (6b) states that there are two things which are not in a person's possession but the Torah puts them into his possession: Chametz on Pesach and a pit in the public domain. However this is not a contradiction to the Machaneh Efrayim, because -- while the Torah has the power to give people presents -- the Chachamim possess a more limited power and cannot make a Kinyan to give things to somebody else.
Kol Tuv,
Dovid Bloom