Here we'll deal with other situations where two men's wives had babies and the babies got mixed up in terms of Pidyon HaBen (redemption of the firstborn). How do we let everything play out? The Mishnah says: There were two women who were married to two men. One of those women had given birth previously, and the other had never given birth. They gave birth to two boys, and the children became mixed up. We don't know which boy is the son of which mother. The man whose wife had never given birth has to give five Sela'im (sela coins) to the Kohen (priest). Even though he doesn't know which boy is his son, he does know that his wife is now giving birth for the first time and that she gave birth to a boy. Therefore, he's obligated in the mitzvah (commandment) of Pidyon HaBen. But if the two women gave birth to a male and a female between them and the babies got mixed up, we don't know who gave birth to the male and who gave birth to the female. There's nothing here for the Kohen. There's no one who has to do the mitzvah of Pidyon HaBen. We don't know if there's a definite Bechor (firstborn son)—a boy over here. Maybe the one who gave birth previously is the one who had the boy, and the one who had not given birth yet had the girl. Therefore, we don't have certainty about the mitzvah of Pidyon HaBen, and we don't go through with it in that case.
We're now transitioning to a new part of the Mishnah, which is talking about the laws of a definite Bechor. If the firstborn son, the definite Bechor, died within 30 days of birth, even if the father already gave the five Sela'im to a Kohen to redeem the child, the Kohen has to return the five Sela'im to the father. The Bechor died within 30 days, and therefore the father was never obligated to do the mitzvah of Pidyon HaBen. So the Kohen has to return the money that he took against the rule, and he's not allowed to keep it. Even if the father didn't give the money to the Kohen, he has to give it now. Once the boy lives for 30 days, the mitzvah of Pidyon HaBen takes effect. The father owes those five Sela'im to the Kohen. Even if the child is no longer living, the father is forced to pay that money. It's as if he died on the previous day, the 29th. The boy did not live the full 30 days, and therefore the father is not obligated to pay the five Sela'im and do the mitzvah of Pidyon HaBen. If the father did not give the five Sela'im to the Kohen, he does not need to give them now. If he already gave them to the Kohen, then the Kohen would have to return them.
Another opinion says that if the child died on the 30th day, and the father already gave the money to the Kohen, he does not get it back. But if he didn't give it to him yet, he does not need to give it. According to Rabbi Akiva, we have a Safek. It's a matter of a doubt whether the obligation of Pidyon HaBen applies on the 30th day or on the 31st. Therefore, whoever is holding the money keeps it. If the father didn't give the money yet, he could claim, “I don't have certainty that I should give it to you.” And you don't have—the Kohen doesn't have—the ability to take it from the father, because maybe the mitzvah is on day 31 and the son died on day 30, and therefore the father would not be required to pay. But if the father already gave the Pidyon money to the Kohen, the Kohen can claim that the mitzvah takes place, or the obligation takes place, on day 30, and the child died on day 30, and therefore the mitzvah was given rightfully. So the father at that point couldn't ask for a refund. So we have a matter of Safek. We can't prove which way is right, and therefore whoever is holding the money is the one who keeps it.
If the father of the Bechor died within 30 days of the child's birth, we don't know whether he did the mitzvah of Pidyon or not. The child is presumed not to have been redeemed. People don't usually do the mitzvah of Pidyon before the requirement hits. So the boy is obligated to redeem himself when he grows up, unless he has proof—some sort of proof that his father did redeem him. But if the father died after the son was 30 days old, the child is presumed to have been redeemed. We can assume that the father went through with it, unless people have some sort of proof or tell him that he wasn't redeemed. People tell the son that his father told them on his deathbed that he never did the mitzvah of Pidyon HaBen, and then the son is obligated to go through with it. So the presumed chazakah, the status of the child, changes depending on how old he was when his father passed away.
He will have a case where a Bechor was born to a father who himself was a Bechor but never had the mitzvah done to him. His own father never did the Pidyon, and then his father died. So the baby's father has an obligation to redeem himself and his son. The father needs to do the mitzvah of Pidyon for himself, and he also needs to do it for his son. But the father has only five Sela'im, not enough money to redeem both of them. Hu kodem et beno—the father takes precedence over his son. Rabbi Yehuda says, Beno kodmo—the son takes precedence over him—she-mitzvato al aviv, because the mitzvah of redeeming himself was his father's mitzvah, and the mitzvah of redeeming his son is his own mitzvah. According to Rabbi Yehuda, here, the mitzvah of Pidyon HaBen for him, the father, was on his own father. It was the mitzvah of Pidyon HaBen, and he's the ben (son) over here. Therefore, in this case, he should redeem his son with those first Sela'im that he has.