Redeeming The Fruits

Tosfos Berachot (35a) says כרם רבעי בזמן הזה מחללין על שוה פרוט' ושוחקו ומטילו לנהר וכן מפורש בשאלתות דרב אחאי פרשת קדושים סימן ק.  Rabbi Akiva Ager asks how can Tosfos say you can redeem רבעי on the value of a perutah, ותמוה לי כיון דילפינן חלול חלול ממעשר ליבעי חלול על דבר שעליו צורה וצ"ע?  

There are a few answers to R.A.E.'s question. The simplest  answer is that the question is right and when Tosfos says שוה פרוטה he means a פרוטה (see Tosfos Bechorot 30a.)

Another approach is that there are two aspects to the law of רבעי.  One law is that there is an issur on the fruits unless they are redeemed or eaten in Yerushalayim and that issur is a carryover from the issur arlah of the first three years.  Another aspect is that there is kedusha on the fruits like maaser sheni.  Tosfos holds in chutz la'aretz since there is no law of masser sheni, the aspect of the fruits being like maaser sheni with kedusha on the fruits doesn't apply and what remains is the issur on the fruits.  According to this approach Tosfos is talking specifically about the fruits of chutz la'aretz. 

Another approach is based upon the opinion of the Rambam (מעשר שני ב:ג ומאכלות אסורות י:יז) that one can exchange (even maaser sheni) onto non money nowadays since there is no obligation to bring the fruits to Yerushalayim there is no obligation of כסף צורה.  The explanation of this difference would be that for the halacha of the act of redeeming the fruit, it is like any other kinyan and there is no need for כסף צורה.  The law of כסף צורה is a law in the process of bringing the maaser sheni to Yerushalayim as the possuk says וצרת הכסף בידך והלכת אל המקום אשר יבחר ה, the צרת הכסף is a method of והלכת אל המקום אשר יבחר ה, no Mikdash, no need for כסף צורה. 

(From Afikay Mayim volume 4 siman 16, Shiurim of Rav Solovetchik on Berachot, Rav Dov Schwartzman Bava Kammah.)