Тосефта к Бамидбар 3:13

One who purchases the fetus of a donkey^*i.e., the ownership rights to the future offspring of a pregnant donkey belonging to an idolater, and one who sells [such a fetus] to him, even though he is not permitted to do so, and one who partners with him, [and] one gives it to him in partnership, and one who receives it from him (i.e., to look after in exchange for a share in the offspring), and one who gives it to him in receivership, [are all] exempt from [the laws] of the first-born, as it is said (Num. 3:13), "[I sanctified to Myself the firstborn] in Israel," but not "in others." And so too you may say regarding priests and Levites, given that priests and Levites are liable in the [laws of the] first-born [in the case of] clean livestock, perhaps they are also liable in the [laws of the] first-born [in the case of] unclean livestock, therefore the verse states Num. 8:17), "of man and of livestock." That which you have with man (i.e., redemption), so too you have with livestock. That which you lack with man, so too you lack with livestock -- to exclude Levites from [the obligation to redeem] the first-born of unclean animals. But the [payments for the] redemption of the first-born son and the redemption of the first-born donkey are only given to priests.