C. F. Keil and Franz Delitzsch on Leviticus 27:9-10

Lev 27:9-10

When animals were vowed, of the cattle that were usually offered in sacrifice, everything that was given to Jehovah of these (i.e., dedicated to Him by vowing) was to be holy and not changed, i.e., exchanged, a good animal for a bad, or a bad one for a good. But if such an exchange should be made, the animal first dedicated and the one substituted were both to be holy (Lev 27:9, Lev 27:10). The expression “it shall be holy” unquestionably implies that an animal of this kind could not be redeemed; but if it was free from faults, it was offered in sacrifice: if, however, it was not fit for sacrifice on account of some blemish, it fell to the portion of the priests for their maintenance like the first-born of cattle (cf. Lev 27:33).