Berochot 11: When Does Possession of a Designated Lamb Take Place?

In daf 9, we learned that if the lamb set aside to redeem a firstborn donkey were to die, one is permitted to benefit from the animal's carcass.  Today's Gamara consider other possibilities.  The first is if the lamb died while it was in the possession of the priest and the Mishna is teaches that the priest is permitted to benefit from the animal, it would be obvious for it is his own money.  The second is if the lam died white in the possession of the owner and that the priest is permitted to benefit from it, this would also appear to be obvious.  The Gemara concludes that as long as the animal has not reached the priest's hands, he does not really possess it.  Thus from the time that the owner has set aside an animal, it is already considered to be in the possession of the new owner.

In our next Mishna in daf 12,  the Chachamim agree with the opinion that as soon as one sets aside the lam, even if it dies, the owner is not responsible to replace it.  Tosafot rule that that the blessing on this mitzvah, "... Who has sanctified us with His mitzvot and commanded us to redeem a firstborn donkey", is actually recited when the lamb is set aside and not when the trade occurs.

According to the acharonim, it is forbidden to derive benefit from the donkey before the redemption takes places.  Thus we should consider the lamb to be a replacement for the donkey, and the lamb should remain forbidden until it is accepted by the priests.  Because this is not the Halacha, the lamb is viewed as possession of the priest immediately after being set aside.