C. F. Keil and Franz Delitzsch on Leviticus 27:26-27

Lev 27:26-27

What belonged to the Lord by law could not be dedicated to Him by a vow, especially the first-born of clean cattle (cf. Exo 13:1-2). The first-born of unclean animals were to be redeemed according to the valuation of the priest, with the addition of a fifth; and if this was not done, it was to be sold at the estimated value. By this regulation the earlier law, which commanded that an ass should either be redeemed with a sheep or else be put to death (Exo 13:13; Exo 34:20), was modified in favour of the revenues of the sanctuary and its servants.