Albert Barnes Commentary Leviticus 27:8

"But if he be poorer than thy estimation, then he shall be set before the priest, and the priest shall value him; according to the ability of him that vowed shall the priest value him." — Leviticus 27:8 (ASV)

If he is poorer than your estimation - Too poor (to pay) your valuation. Compare (Leviticus 27:7), (Leviticus 27:11).