Nedarim 33b - Mavriach Ari

We already discussed this somewhat in kesubos 107 http://hearos.blogspot.com/search?q=mavriach but there is more to talk about.

The Ran assumes that we need the sevara of mavriach ari for all 3 cases listed in the mishna. The gemara discusses paying a debt for someone who is assur to receive hana'ah from you. The heter is either based on the opinion of Chanan who says that when reuven pays shimon's debt on the condition that shimon will owe him money, shimon does not owe money to reuven since he never requested the loan, therefore when he does it on the condition of mechila it is not considered benefiting shimon - that is called mavriach ari, meaning he simply prevents shimon from suffering a loss. The other possibility is that we are speaking about a case where shimon doesn't actually owe anything since he stipulated with the lender not to collect, so when reuven pays the "debt" for him, shimon is not receiving any real gain.

The Ran understands that by being paying his shekel we can also use the concept of mavriach ari according to Chanan. However, it is slightly more complicated because aside from the concern of getting the gizbar (who is like the lender) off shimon's back, reuven is buying shimon a share in korbanos. Therefore the Ran has to say that even if shimon does not pay, he is still entitled to a share in the korbanos (as rashi writes in kesubos, not like tosafos there).

Being that the Ran holds that mavriach ari applies to the paying the shekel as well, he must hold that mavriach ari applies even to a situation where the damage is definite. This is to the exclusion of Tosafos who writes that mavriach ari only applies when reuven saves shimon from the fear of a lion but not from an actual lion. Tosafos holds that if the damage is definite then reuven cannot protect shimon from it, since it qualifies as providing him benefit. Tosafos can only say this because they understand the case of paying the shekel is speaking according to everyone where shimon already payed and it was lost, but if shimon has not yet payed then reuven cannot pay for him because shimon has no way out of the situation, it is a definite loss and therefore is not called mavriach ari. The Ran clearly disagrees with Tosafos and holds that even when the damage is definite such as paying his shekel that was never payed for, we can apply mavriach ari.

It is not so clear how the Ran will answer Tosafos question from the case in Baba Metzia where a shepherd saves the herd from a lion attack and we require the owner to pay the shepherd and do not consider it mavriach ari. Why would we consider the case of paying the shekel to be mavriach ari? See first answer of Tosafos B.M. 31b for a possible answer, but it needs explanation.