Midras Imamat 27:29: Sifra & Mekhilta DeRabbi Yishmael

7) (Vayikra 27:29) "Every cherem which is devoted of man shall not be redeemed, (for) he is to die." Whence is it derived that if one awaiting execution said: "My value upon me" (to give to the Temple), he has said nothing? From (He is) "cherem ['condemned'] … he shall not be redeemed." This tells me only of (those who are guilty) of severe capital offenses, (where unwitting perpetration is not susceptible of atonement.) Whence do I derive (the same for) lesser capital offenses (where unwitting perpetration is susceptible of atonement)? From "Every cherem … shall not be redeemed." I might think that this is so (even) before his verdict has been pronounced. It is, therefore, written "which is devoted 'of man' (connoting one whose verdict has been pronounced) shall not be redeemed," and not one whose verdict has not been pronounced.

8) R. Chananiah b. Akavya said: He is valuated (ne'erach) because his worth (i.e., the worth of one who is valuated) is fixed (by Scripture), but he cannot be made the subject of a vow (nidar) because his worth is not fixed by Scripture. R. Yossi says: He can vow and valuate and consecrate, and if he damages, he is liable for payment.

R. Akiva says: the value of the killer (i.e., the owner of the ox). And thus do we find that redemption is not given for those who are put to death. In all places, those who are liable to death at the hands of man are not redeemed, as it is written (Leviticus 27:24) "Any cherem that is devoted from a man (going out to be executed) shall not be redeemed, (for) he is going to be put to death" (and has no valuation). But here "he shall give the redemption of his soul."