Masechet Temurah discusses what happens if one makes a temurah -- attempting to designate a replacement for a dedicated offering. Though this is not allowed, the kedushah of the original does (in many cases) fall on the replacement and the person is obligated to offer the replacement as a sacrifice.
On 20b there is an objection to one particular instance, a person cannot receive atonement through the temurah offering because "דאין אדם מתכפר בדבר הבא בעבירה" a person cannot get atonement through committing a sin (making the temurah exchange).
But if the temurah is inherently a sin, how can anyone, in any case offer it as a sacrifice -- isn't offering a sacrifice a mitzvah (according to the list of 613, many parts of the service are discrete mitzvot). Even simply offering the temurah as a consequence is, in and of itself a mitzvah (in at least a rabbinical sense).
Shouldn't this answer of "a mitzvah that comes about my means of committing a sin" excuse a person from having to offer any temurah as a sacrifice? Or does he offer it and somehow it isn't fulfilling any mitzvah and the action is only a punishment? If one eats any of the temurah and makes a bracha, wouldn't this be a bracha whose fulfillment comes because of the sin?
Does the concept of mitzvah haba b'aveiragh not apply here? If so, then why does "ein adam mitkaper"?