QUESTION: The Gemara asks that one should not be permitted to purchase a Lulav of Shevi'is from an Am ha'Aretz, because the money that the Am ha'Aretz receives for the Lulav becomes sanctified with Kedushas Shevi'is.
In what way does Kedushas Shevi'is affect a Lulav, which is not an edible fruit? RASHI (DH Ta'ama) explains that Kedushas Shevi'is affects a Lulav in the event that one exchanges a Lulav of Shevi'is for money. The money attains the Lulav's Kedushas Shevi'is, and it must be used to buy only edible food and not a durable item (like a garment).
However, Rashi earlier (39a, DH Ein Mosrin) writes that the only reason why Sechorah (commerce) with fruits of Shevi'is is forbidden is because one is required to consume them ("l'Ochlah") or the money exchanged for them before the time of Bi'ur arrives. Accordingly, the reason why one may not buy a garment with money that has Kedushas Shevi'is (money that was used to buy fruit of Shevi'is) is because the garment will remain in existence past the time of the fruit's Bi'ur, as Rashi explains later (41a, DH u'Vikesh). According to this explanation, there is nothing wrong with conducting Sechorah, or purchasing garments, with a fruit of Shevi'is that has no time of Bi'ur.
Rashi later (40b, end of DH Shema Yigadel) writes that the time of Bi'ur of a fruit is determined by the time at which that fruit no longer is found naturally in the fields. A Lulav, however, remains on the tree perennially (it does not rot and fall off like fruit). Indeed, it is presumably for this reason that Rashi here (40a, DH Ta'ama) does not write that Kedushah of Shevi'is affects the Lulav in that the Lulav must be used up before its time of Bi'ur.
However, if the Lulav does not have any time of Bi'ur, then why is one prohibited to buy a garment with the money exchanged for a Lulav of Shevi'is? Since there is no time of Bi'ur for a Lulav itself, an object bought with its value should also be allowed to remain. One should be permitted to use the money to buy a non-edible item, according to Rashi. (TOSFOS 39a, DH she'Ein)
ANSWER: Rashi maintains that a Lulav does have a time of Bi'ur. Although Lulav branches remain on the tree throughout the year, there are seasons during which no new sprouts are produced, and the older Lulav branches open up and their leaves spread out. When its leaves spread out, the Lulav is no longer fit to be used as a broom, its primary utilitarian purpose (Rashi DH Yatz'u). It is at that moment that the Lulav is no longer considered available in the natural environment.
If a Lulav has a time of Bi'ur, then why does Rashi not write that the practical application of Kedushas Shevi'is to a Lulav is that one must consume the Lulav before its time of Bi'ur? The answer is that Rashi's intention here is to explain why one may not give the value of a Lulav to an Am ha'Aretz. Rashi needs to show how the Am ha'Aretz will misuse the money that he receives from the sale of the Lulav (see Rashi to 39a, DH Ein Mosrin). Rashi therefore mentions a practical application with regard to the value of the Lulav.
According to Rashi, it seems that money exchanged for fruits of Shevi'is becomes sanctified with Kedushas Shevi'is only with regard to the requirement to consume that money before the time of Bi'ur (but not with regard to the other laws of Shevi'is). This explains why the Gemara is concerned only that the Am ha'Aretz will not consume the money before the time of Bi'ur arrives, and it is not concerned with any of the other ways that items of Shevi'is might be misused (such as being wasted (Pesachim 52b), being used to make a bandage or fed to an animal (Shevi'is 8:1), being used as payment for one's debts (Shevi'is 8:4 and Avodah Zarah 62a, as cited by Rashi later, 44b, DH Amar Lei)). Rashi maintains that the Kedushas Shevi'is is not transferred from the fruits onto the money ("Tofes Damav") with regard to the other prohibitions, but only with regard to the requirement of Bi'ur and the prohibition against giving more than three meals' worth of money to an Am ha'Aretz (which is related to the law of Bi'ur, according to Rashi). (M. Kornfeld)