פירוש על קידושין 106:2

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.