I don't understand how a value of a peruta is derived from the verses cited as the minimal value of a returnable object. Is this an arbitrary value d'rabbanan?
Fred Stone, Red Bank, NJ, USA
The Perutah is a "Shi'ur," or required amount, that pertains to most laws with monetary ramifications. With regard to "Shi'urim" in general, the Gemara teaches (Sukah 5b, Eruvin 5a) that Shi'urim are a "Halachah l'Moshe mi'Sinai," i.e. they are part of the Mesorah, and not based on derivations from verses. Nevertheless, there is a logic behind choosing this amount as the Shi'ur: a person normally does not make a big deal about sums of less than a Perutah (see Sanhedrin 57a). Therefore objects worth less than a Perutah are not considered objects of monetary significance.
In some places, Rishomim write that Shi'urim were "given over to the Chachamim to choose and designate [as they believe the Torah meant the Shiur to be]" (See Tosfos Yoma 79b DH Lomar)
Mordecai Kornfeld