Gem. in Bechoros 9b explains R. Shimon's view saying, the first-born donkey must be permitted since the redeeming lamb is permitted and it is impossible to have a situation in which the redeemed item is (or was) forbidden but the redeemer is permitted. Then the Gem. questions this principle based on the laws of Sheviis where, if the Sheviis-produce is exchanged, the new item is permitted and the original Sheviis-produce remains forbidden. I don't understand. By Sheviis-produce there is no redemption just an exchange and indeed an exchange which doesn't accomplish anything. The Gem. might as well ask from any issur'hanaha when it is sold, the original item remains forbidden but the money is permitted.
Michael Dalezman
The nature of a transaction being done with Sheviis produce shows an effort to redeem the fruit, and the status of Kedushah remains the same by both the money and the fruit. The fact that the desired results of the redemption are "fruitless" does not mean that a redeeming transaction did not take place (i.e. Temurah, where the opposite of the redeemer's intention takes effect).
Your second question is indeed asked by the Turei Even and Rashash. The Rashash answers that in fact this is a misconception. Although the Gemara in Kedushin (56b) states that one can be Mekadesh a woman with money he received from the sales of Isurei Hana'ah such as Orlah, that is only because he is doing the Mitzvah of Peru u'Revu. In general, the Rashash says, proceeds from the sale of Isurei Hana'ah are forbidden mid'Rabanan. The reasoning of our Gemara is that it is impossible to have a situation in which the redeemed item is forbidden but the redeemer is totally permitted, even mid'Rabanan. Being that the sheep in this case is totally permitted, it would follow that the Peter Chamor is also permitted.
Kol Tuv,
Yaakov Montrose