C. F. Keil and Franz Delitzsch on Leviticus 27:8

Lev 27:8

But if the person making the vow was “poor before thy valuation,” i.e., too poor to be able to pay the valuation price fixed by the law, he was to be brought before the priest, who would value him according to the measure of what his hand could raise (see Lev 5:11), i.e., what he was able to pay. This regulation, which made it possible for the poor man to vow his own person to the Lord, presupposed that the person vowed would have to be redeemed. For otherwise a person of this kind would only need to dedicate himself to the sanctuary, with all his power for work, to fulfil his vow completely.