Bechorot 10: Redeeming - and not Redeeming - Firstborn Donkeys

We learn in Shemot (13:13) that if the owner of a firstborn donkey does not wish to redeem it by exchanging it for a lamb, then the donkey is killed by decapitation.  The Gemara teaches a baraita that shares these Rabbi Yehuda's details on this act: he breaks its neck from behind with a kufitz, cleaver, from the back and buries it, and no benefit can come from it.  Rabbi Shim on permits one to derive benefit from it.  "He must not kill the donkey with a reed, nor a sickle, nor an ax, nor a saw, nor starve it in a room a lone, to shear it or use it for labour.  Again, Rabbi Yehuda teaches these rulings while Rabbi Shimon permits it.  

The Gemara decides that that their disagreement refers to a living animal.  The first part refers to monetary benefit and the second part refers to the benefit derived from its body (its wool or work).  Again, Rabbi Shimon permits use of the firstborn prior to decapitation and Rabbi Yehuda forbids deriving such benefit.

In Rambam's Mishnet Torah (Hilchot Bikkurim 12:2) rules along with Rabbi Yehuda and agrees that if the firstborn is sold, the money from the sale is forbidden.  The Ra'avad argues that there are two situations where a forbidden object transfers its status to money for which it is exchanged: avoda zara, idol worship, and shevi'it, agricultural laws.

In his Mishneh Torah, the Rambam (Hilkhot Bikkurim 12:2) rules like Rabbi Yehuda and says that if the firstborn is sold, the money derived from the sale is forbidden. The Ra’avad questions this ruling, arguing that there are only two situations where a forbidden object transfers its status to money for which it is exchanged – avoda zara and shevi’it.

Rabbi Akiva Eger suggests that this case may be unique.  When it comes to firstborn donkeys, deception does not remove the prohibition.  Thus it is permitted to transfer that deception does not remove the prohibition.  

Rashi suggests another possibility regarding monetary benefit.  Payment may be made when the animal is rented.  A sold animal is not mentioned and thus money may not be intrinsically forbidden that a penalty could be imposed on the owner.  Since he wanted to cause a loss to the kohen, he is made to lose out.