Firstlings (Leviticus 27:26-27).
“Only the firstling among beasts, which is made a firstling to Yahweh, no man shall sanctify it; whether it be ox or sheep, it is Yahweh's.”
A firstling cannot be set apart as holy to Yahweh. It is already Yahweh's. See Exodus 13:2. Such a vow would be meaningless.
“And if it be of an unclean beast, then he shall ransom it according to your estimation, and shall add to it the fifth part of it: or if it be not redeemed, then it shall be sold according to your estimation.”
Where, however, the firstling is an unclean beast it can be redeemed at its estimated value plus one fifth. This presumably does not apply to an ass which has to be redeemed at the price of a lamb because an ass was especially valuable (Exodus 13:13).