Leviticus 27:8
But if he be poorer than thy estimation, then he shall present himself before the priest, and the priest shall value him; according to his ability that vowed shall the priest value him. But if he be poorer than thy estimation, then he shall present himself before the priest, and the priest shall value him; according to his ability that vowed shall the priest value him.
Leviticus 27 belongs to the instruction concerning vows and tithes, cited as the law of tithing. Within that chapter this verse concerns the valuation of a person dedicated to the Lord by a special vow.
If the person dedicated is poorer than the priest's standard stated valuation, that person shall present himself before the priest. The one who cannot meet the valuation set for the person vowed must come and appear before the priest. This covers a person covered by the vow who is poorer than the stated valuation, including a household member dedicated by such a vow and someone included in a vow.
The priest shall then value him according to the ability of the poor person valued, what he can afford. The priest assesses the person according to what the person can afford and sets a value according to that person's ability. On one reading, the priest sets the value according to the ability of the one who made the vow.
The dedication is fulfilled through payment or redemption at the priest's valuation rather than by offering the person as a sacrifice or otherwise using the person. A household member thus dedicated could be redeemed at the priest's valuation, and this provision is a way of release from vows. Where what was vowed could not lawfully be offered, its estimated value could be given instead.
What is vowed to God is sacred, and when someone retains vowed property the priest values it and an additional fifth is paid. Thus the verse shows how a vow involving something that could not lawfully be offered as a sacrifice could still be fulfilled. So Jephthah could dedicate his daughter to the Lord in lifelong virginity instead of offering her as a burnt offering.