Halakhah for Numbers 18:16

From the laws of the commandment is that which they, may their memory be blessed, said (Bekhorot 49a) that the commandment of redemption [begins when] the child is thirty days old, for [at that point he is no longer considered a potential] stillborn, as it is stated (Numbers 18:16), "And his redemption that you redeem, from one aged a month." And this commandment is the responsibility of the father (Kiddushin 29a). If the father transgressed and did not want to redeem him, the commandment devolves upon the son, to redeem himself when he becomes an adult. The amount of the redemption is five sela, whether it be money equivalent to five sela or commodities that are the equivalent of money, such that their makeup is money, to exclude slaves, land and deeds - as if one redeemed him with them, he is not redeemed. The five sela may be given to one priest or to several priests, and the obligation is to give it to a male priest, and not to a female priestess; as it is written regarding the money of redeeming the firstborn, "Aharon and his sons." If the priest wishes to return the redemption money after it is given to him, [the father] has [still] fulfilled his obligation, so long as the money is not given with this condition [in mind]. If he did give the money with this condition, the son is not redeemed, until he concludes in his heart to give it to him as a full gift. But if he did specify and gave the money with the condition that it be returned, and the priest was appeased to this, the child is redeemed.