Rashi on Leviticus 27:10/רש"י על ויקרא כ״ז:י׳

1טוב ברע [HE SHALL NEITHER CHANGE IT NOR EXCHANGE IT] A GOOD FOR A BAD — i. e. he shall not give in exchange a perfect animal of a non-sacred character for one devoted to the Sanctuary but having some blemish (cf. Rashi on Temurah 9a on ‎‎‎מתני' טוב ברע),

2‎‎‎או רע בטוב‎ OR A BAD FOR A GOOD — It follows à fortiori that he must not give in exchange a perfect animal for another perfect one, or one with a blemish for another with a blemish (Temurah 9a).