Charles Ellicott Commentary Deuteronomy 15:19

"All the firstling males that are born of thy herd and of thy flock thou shalt sanctify unto Jehovah thy God: thou shalt do no work with the firstling of thy herd, nor shear the firstling of thy flock." — Deuteronomy 15:19 (ASV)

All the firstling males... you shall sanctify — that is, recognize them as the property of Jehovah by not using them for ordinary purposes. In Leviticus 27:26 we read, No man shall sanctify it — that is, shall make it the subject of a special vow or dedication, because it already belongs to Jehovah. This is the only interpretation consistent with the context in Leviticus; for Deuteronomy 27:0 deals entirely with “voluntary” offerings, which are in a sense outside the Law. (See Notes at the commencement of Deuteronomy 28:29).