Deuteronomy 15:19

Firstling males. The law was established at the Exodus from Egypt that all firstborn males of both man and beast belonged to Jehovah (see on Ex. 13:12-15; Num. 18:15-18).

Sanctify unto the Lord. This does not contradict Lev. 27:26, “The Lord’s firstling, no man shall sanctify it.” The meaning here is that the owner shall recognize the firstling as Jehovah’s property, and must under no circumstances use it for any other purpose. Lev. 27:26 means that no man shall take what the Lord already considers as His — the firstling — and present it in payment of a personal vow.

Do no work. The male firstlings were sanctified to Jehovah. They were not to be put to ordinary secular labor, even though they might remain temporarily in a man’s possession. The firstling females were not dedicated to the Lord as were the males. They might be offered as peace offerings, with the offerer partaking of them in a sacrificial meal and giving a portion also to the priests. This is in harmony with the principle of the second tithe (ch. 14:23).

Nor shear. A firstling sheep was not to be shorn, for it was dedicated to a holy use. God has claim on the first of all we possess: our life, our affections, our physical, mental, and spiritual powers, our service, our property.