During the Sabbatical year, fruits that grow are hefker and available to all – men and beasts. As long as the fruits are available on the trees, a person can pick them to eat immediately, or to store for later use. Once the harvest season for a particular fruit is over, and that type of fruit is no longer available on the trees, there is an obligation of bi’ur – “removal” of the fruit.
According to the Ramban in his commentary on the Torah, at the time of bi’ur the person who is storing fruit must remove it from his property and declare it to be hefker. At that time it can be taken and eaten by anyone. The Rambam understands that bi’ur means destruction, and that the fruits of the Sabbatical year can only be eaten as long as they are available in the fields. After that time, the fruits are forbidden and must be destroyed.
Our Gemara contrasts things that are considered davar she-yesh lo matirin (something that is forbidden today, but will become permitted at a later time) and things that are davar she-ein lo matirin (something that is forbidden forever). In the latter case the concept of bittul (nullification when mixed with a larger volume of permitted food) applies, while in the former case, bittul does not apply (since it will become permitted simply with the passage of time, there is no pressing reason to employ the rules of nullification). Thus, the Gemara quotes Rabbi Shimon who lists forbidden foods – like teruma and kilei ha-kerem (grape vines and grain that grew together as a forbidden mixture) – that are nullified when mixed with larger amounts of permitted foods, since they are all davar she-ein lo matirin. On the other hand, such things astevel (produce that is forbidden because it has not yet been tithed, which will be permitted after tithing) or hadash (grain that is harvested before Pesaḥ that is forbidden until after the first day of Pesaḥ when the korban ha-omer is brought) are considered davar she-yesh lo matirin and will not become nullified.
With regard to Sabbatical fruits, Rabbi Shimon is quoted by the Gemara as saying that they are considered a davar she-yesh lo matirin that cannot become batel (nullified). This explanation works well according to the Ramban, who believes that after the time of bi’ur the fruits can become permitted by declaring them ownerless. According to the Rambam, who believes that such fruits become forbidden forever, we will have to interpret the case like the Ran, who suggests that Rabbi Shimon must be referring to the time prior to bi’ur, saying that any mixture that includes shemitta fruits must be eaten before that time.