Karetot 25: When the Asham Taluey is Obligated or Optional

We know that there are three ways to use the law of asham taluey, provisional offerings.  These are:

• Rabbi Eliezer: an asham taluey can be donated at any time whether or not one knows they have sinned
• Bava ben Buta: an asham talen cannot be donated unless there is a possibility that one has sinned
• The Sages rule that an asham taluey is only brought when one believes that s/he might have transgressed a sin which is punishable by karet

The question here is whether or not an asham taluey is understood to be an obligatory sacrifice or a sacrifice that is optionally donated.  From the Gemara in Chullin (41b) we learn that if we slaughtered an animal today without an altar in our Temple, we ignore the person's intention and the animal is kosher for consumption.  This is only the case when one believed his/her sacrifice to be obligatory.  Otherwise s/he would have seemed to have sacrificed outside of the Temple area.  As well, in Massechet Chullin, we are told that one can bring an asham taluey as a sacrifice.  This is Rabbi Eliezer's opinion in today's Mishna.