פירוש על קידושין 106:10

When Jeremiah uses the words “to the Lord” and “holy” he is referring to Israel, not to terumah.
The Talmud then asks if Israel is holy to the Lord then why shouldn’t terumah also be holy to the Lord, and not usable as betrothal money. After all, the two are compared.
Rabin now pulls out another element of the verse about tithe—tithes are “for the Lord” and must remain in their natural form. They cannot be used for betrothal.

Sanctified property can become non-sanctified property if it is redeemed. However, if it is not redeemed then it remains sanctified and cannot be used for betrothal. According to Rabbi Meir, if the man intentionally uses sanctified property as his betrothal money, he is in essence redeeming it. The betrothal is valid and the man will owe to the Temple the value of that which he gave to the woman. However, if he does so unwittingly, then the sanctified property is not redeemed and therefore the betrothal is invalid.
Rabbi Judah disagrees on both counts. He holds that one who intentionally uses the sanctified property for betrothal does not thereby redeem it, therefore the betrothal is invalid. However, if he unwittingly uses the sanctified property this is considered “me’ilah”—improper use of sacred property. In such cases the object which was misappropriated loses its sacred status and the person who misappropriated the property owes the Temple the value of the object plus one-fifth and must bring a guilt offering. The key for our purposes is that the object is no longer sacred, and therefore the betrothal is valid.

R. Judah had said that if a man unwittingly uses hekdesh for betrothal, she is not betrothed. R. Meir said that if a man unwittingly uses second tithe money, she is not betrothed.
R. Yohanan gives two reasons for why not, but R. Jacob does not remember which goes with which halakhah. Yes, this is a confusing one.

R. Judah had said that if a man unwittingly uses hekdesh for betrothal, she is not betrothed. R. Meir said that if a man unwittingly uses second tithe money, she is not betrothed.
R. Yohanan gives two reasons for why not, but R. Jacob does not remember which goes with which halakhah. Yes, this is a confusing one.

R. Yirmiyah tries to solve the puzzle. The woman does not want to be betrothed through tithes because she’d have to bring them to Jerusalem to use them there. However, there is no reason why the husband won’t want to betroth his wife through tithes. He gets a wife without having to use other money.
When it comes to hekdesh, neither want to accidently transform hekdesh into non-sacred property. People are not supposed to do this unless they are specifically redeeming the hekdesh.

R. Ya’akov argues the opposite (of course he does—this is just that kind of sugya). Neither want to use tithes for betrothal. She does not want to because of the trouble of the journey. He does not want to lest she loses the second tithes on her journey to Jerusalem and she derives no benefit from them. This will cause her distress, that she received no benefit from her betrothal money.
When it comes to hekdesh, he might be okay that she is betrothed with hekdesh. After all, he gets a wife without spending any extra money.
Thus the sugya ends without us really knowing why each tanna said what he said.

Since the act of betrothal is not valid, the money retains its sacredness.

If a person tries to use sacred money to buy something, the acquisition is not successful.

R. Hiyya b. Avin raises an objection against R. Hisda. The discussion in this baraita is of a person who deposits some money with someone and does not tell him that the money is sacred. If he deposits money in a bundle with a money-changer, the money-changer may not use the money. If he does use it, and it is sacred, then he has committed trespass, because he should not have been using the money. But if not bundled up, the money-changer may use the money, and therefore, he has not committed trespass. Rather the depositor has. If one deposited it with a homeowner, the homeowner may never use the money, whether it is bundled up or not. The tannaim disagree about the status of a storekeeper—is he treated like a homeowner (R. Meir) or like a money-changer (R. Yehudah). But all agree that if the storekeeper used the money, he has trespassed. This implies that the transaction takes effect, meaning that if hekdesh is used to buy goods, the transaction is successful.

R. Hiyya b. Avin raises an objection against R. Hisda. The discussion in this baraita is of a person who deposits some money with someone and does not tell him that the money is sacred. If he deposits money in a bundle with a money-changer, the money-changer may not use the money. If he does use it, and it is sacred, then he has committed trespass, because he should not have been using the money. But if not bundled up, the money-changer may use the money, and therefore, he has not committed trespass. Rather the depositor has. If one deposited it with a homeowner, the homeowner may never use the money, whether it is bundled up or not. The tannaim disagree about the status of a storekeeper—is he treated like a homeowner (R. Meir) or like a money-changer (R. Yehudah). But all agree that if the storekeeper used the money, he has trespassed. This implies that the transaction takes effect, meaning that if hekdesh is used to buy goods, the transaction is successful.

The Talmud reads R. Meir as only arguing against R. Yehudah, not expressing his own opinion. According to R. Meir’s own opinion, no matter what, the storekeeper has not committed trespass. The money remains holy and the goods are not acquired. In the dispute, he is speaking to R. Yehudah—to you, the shopkeeper should be like a private individual. He should never be allowed to use the money. R. Yehudah responds that he is like a money-changer. If the money was not bundled up, then he may use it and he would not be liable for trespass.

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.