Torah Temimah cites Temurah 15a.
The Sages taught: From where that one who dedicated his chatas and it was lost, and he designated another in its place, and then the first was found and both stand — from where that he may bring whichever he prefers?
Ribbuy derived from the optional language of the sheep-chatas passage — owner may choose which of the two animals to bring.
When both the original and the replacement chatas animals are available, the owner may bring whichever he prefers. The Torah’s optional formulation for the sheep chatas supplies a ribbuy that leaves the choice with the owner rather than forcing him to use only the first or only the second animal.