Shevi'is, Chapter 1, Mishnah 3. To understand our mishnah we must first look ahead to a mishnah later in the tractate, in the ninth chapter, where Rabbi Yehudah and Rabbi Yose disagree about the law of the mitzvah of bi'ur (removal).
The Mitzvah of Bi'ur and the Dispute of the Tanna'im:
Once a particular fruit or vegetable of the seventh year is no longer to be found in the field, a person must rid himself of it even if it is sitting in his home. That is bi'ur. But the Tanna'im disagree as to what bi'ur consists of and how it is carried out:
Rabbi Yehudah: After the time of bi'ur, only the poor are permitted to eat the produce. The owner must declare the produce ownerless, and from that point on only the poor - only others - may acquire it.
Rabbi Yose: Anyone may acquire it. Once he has declared it ownerless, anyone is free to go and acquire it, including the owner of the field himself.
The Words of the Mishnah:
"Luf she'avar alav shevi'is" - luf, that type of onion we dealt with in the previous mishnah, was planted in the sixth year and remained in the ground throughout the sixth year, at the end of the sixth year, through the seventh year and into the eighth year. It therefore contains a mixture of seventh year growth and growth that is not of the seventh year.
Rabbi Eliezer's Position:
"Im liktu ha'aniyim es alav - liktu" - if the poor gathered the leaves of the luf that grew during Shemittah, they have taken ownership of them and acquired them. "Ve'im lav - ya'aseh cheshbon im ha'aniyim" - and if they did not acquire them, the householder, who is permitted to eat those parts of the luf that did not grow during Shemittah, must make an accounting with the poor as to which portion ought to be theirs, since the luf is subject to the laws of bi'ur.
Rabbi Eliezer holds like the opinion in the ninth chapter that after the time of bi'ur the householder himself cannot acquire the produce, and only the poor - only those who are not the owners - can acquire it. Accordingly, the poor own one portion of the luf and the householder owns the other portion, and they must work out an accounting of who owns each portion and compensate one another for it.
Rabbi Yehoshua's Position:
Rabbi Yehoshua, by contrast, holds like the opinion in the ninth chapter that the householder too may eat the produce. Therefore: "Im liktu ha'aniyim es alav - liktu" - if the poor got there first and seized the opportunity and gathered those leaves that grew during Shemittah, they are theirs. "Ve'im lav - ein la'aniyim alav cheshbon" - and if they did not gather them, the poor have no claim at all, for once the householder declared the luf ownerless and thereby performed the act of bi'ur through his declaration of hefker, he may go back and acquire it for himself, and the poor have no special claim to the leaves of this luf - even to those parts that grew during the seventh year.
In summary: The mishnah deals with luf over which the seventh year has passed, which contains a mixture of sixth year growth and seventh year growth, and the dispute between Rabbi Eliezer and Rabbi Yehoshua depends on the dispute in the ninth chapter regarding the nature of bi'ur. According to Rabbi Eliezer, who holds that only the poor acquire the produce after the time of bi'ur, if the poor did not gather the leaves, the householder must make an accounting with them; and according to Rabbi Yehoshua, who holds that the owners too may go back and acquire the produce after it has been declared ownerless, if the poor did not gather the leaves they have no accounting with him at all.