A question raised by the aharonim is why we view this situation solely as a monetary obligation. Surely, aside from the obligation to pay the kohen five sela, there is also an additional aspect of fulfilling the commandment of pidyon ha-ben – the mitzvah of redeeming the firstborn – and we ordinarily rule that in situations where there is the possibility of a biblical obligation, we are stringent and require that it be fulfilled. One response is that the commandment is to pay the kohen five sela, creating a monetary obligation. In the event that there is no monetary obligation, the commandment does not apply.
What if twins are born and we cannot tell which child is the firstborn?
The Mishnah on today’s daf (=page) teaches:
If a man’s wife had never before given birth and she gave birth to two males, he gives five sela to the priest. If one of them dies within thirty days of birth the father is exempt. If the father dies and the sons survive, Rabbi Meir says: If they gave the five sela before the property was divided up, it is irrecoverable but if not, they are exempt. But Rabbi Yehudah says: There is a claim on the property. If she gave birth to a male and a female, the priest receives nothing.
According to this Mishnah it appears that in cases where it is not clear whether or not there is an obligation to pay the five sela redemption fee to the kohen, there is no obligation to pay. Thus, a father whose wife gives birth to twin boys knows that he must pay the kohen, even if they are unsure which child was born first. If one of the twins dies or if the twins were a boy and a girl, then it is not clear that the living boy was the firstborn, and no redemption must be paid.
Rashi explains that this ruling is based on the principle in money matters that ha-motzi me-haveiro alav ha-ra’ayah – someone who believes that his fellow owes him money must bring proof to his claim. In cases where the kohen cannot prove that the boy is the firstborn, there is no obligation to pay him the five sela.