1) Reb Chaim, I think you may be referring to the Sefas Emes to Temurah 8a (DH Meisivei Behemtah). He writes there that in order to be able to effect Temurah, one does not not have to be the owner of the animal. If a tenth animal (Ma'aser) is born into the flock owned by the Yisrael, the owner of the herd can effect Temurah by swapping the Ma'aser animal with another animal, even though he does possess permanent ownership of the Ma'aser animal, since it must be given to the Kohen. The Yisrael cannot sell the tenth animal, but he is capable of swapping it with another animal and making Temurah. Ma'aser Behemah is only momentary property, but he is considered to possess a sufficient level of ownership to make it possible for him to effect Temurah.

2) The Shitah Mekubetzes (addenda 7b, #15) writes something similar. If a Yisrael possesses a Bechor which he is obligated to give to a Kohen, he is considered the owner of the Bechor and can do Temurah. This is because he can give the Bechor to whichever Kohen he wants, so this is a sufficient level of ownership to make it possible for him to effect Temurah even though the Bechor could not always remain as his property.

Kol Tuv,

Dovid Bloom