Mishnayos Bechoros Perek 8 Mishnah 8

אין פודין – [they do not redeem] the firstborn [male] of a man.

לא בעבדים – even though that we said above (i.e., in the previous Mishnah) with silver or with the equivalent of silver, we don’t redeem [a firstborn male child] with servants.

ולא בשטרות – that if he has a document on his fellow for five Selaim [owed him] and he gave it to the Kohen, that he (i.e., the Kohen) should collect that obligation/liability with the redemption of his son, his son is not redeemed.

ולא בהקדשות – that is to say, and not with that which is sanctified [to the Temple] also we don’t redeem them neither with slaves, documents nor with land.

כתב לכהן שהוא חייב לו ה' סלעים – because of the redemption of his [first-born] son.

חייב ליתן לו ובנו אינו פדוי – it is the word of the Torah that his son is redeemed when he gives [the money]. And what is the reason that they said that he must give it and his son or else his son is not redeemed? It is a decree perhaps they will say that the redeem with documents.

לפיכך – that we said that he is required to give him (i.e., to the Kohen) five other Selaim for the redemption of his son.

אם רצה הכהן – [if the Kohen wanted] to return them and to give them (i.e., the five Selaim) to him (i.e., the father) as a gift, he is permitted, but there is no other remedy/ordinance. But even though that the Kohen is permitted to return and give the five Selahim as a gift to the father of the first-born [son], if it is the intention of the father of the son relies that the Kohen will surely return to him the five Selaim, the son is not redeemed, whether the Kohen returned them or whether he didn’t return them. This is how it appears from the Gemara (see Talmud Bekhorot 51b).

שנאמר יהיה לך ופדה תפדה – when the redemption will be yours, then your son is redeemed, and this verse (Numbers 18:15) is stated to Aaron.