פירוש על קידושין 107:1

According to Rav, R. Meir usually holds that if one uses hekdesh unwittingly, the hekdesh does not remain sacred. And if he uses it intentionally then it does not become non-sacred.
However, this is exactly opposite of what R. Meir says about using hekdesh for betrothal in our mishnah. In the mishnah he said that if one used it unwittingly the betrothal is invalid and the hekdesh remains sacred. If used intentionally, the betrothal is valid and the hekdesh becomes non-sacred. Since Rav believes this mishnah to be different from R. Meir’s usual opinion, he interprets it as referring to a priest who betroths with his priestly clothing. If he does so unwittingly (I know this is hard to imagine) then she is not betrothed and the clothes remain sacred. This is because the priests may benefit from their clothing even when not performing the worship service. It would simply be too hard to tell the priests that they cannot even wear this clothing when not serving in the Temple and the Torah was not given to angels.
If he intentionally uses them for betrothal, they become non-sacred and she is betrothed.