Bechorot 48: Firstborn,Twins and Payment

A new Mishna teaches that if a man's wife had never given birth before but then gave birth to two male children, he gives five seal to the priest.  If one of them should die within thirty days of birth, the father is exempt.  If the father dies and the sons survive, Rabbi Meir says that if the five seal was given before the property was divided up, in is irrecoverable but if not, they are exempt.  Rabbi Yehuda argues that there is a claim on the property.  If she gave birth to a male and a female, the priest receives nothing. 

The Gemara debates whether anything questionable when it comes to twins means that the firstborn is not redeemed.  Even if they do not know which male child of two males is the firstborn, one must be redeemed (as long as one lives).  Rashi suggests that this is based on a monetary matter: ha'motzi m'chaveiro alav ha'ra'aya, someone who believes that his fellow owes him money must bring proof to his claim.  If the priest cannot prove that the boy is the firstborn, there it is not required to pay the five sela.

The acharonim wonder why we look at this as a monetary obligation.  We are also required to do the mitzvah of pidyon ha'ben, redeeming the firstborn, and we ordinarily rule that biblical obligations are absolutely required.  Another response is that the commandment to pay the priest five sela creates a monetary obligation.  When there is no monetary obligation, the commandment does not apply.