Rashi on Leviticus 27:8/רש"י על ויקרא כ״ז:ח׳

1ואם מך הוא BUT IF HE POOR so that his means are not sufficient to pay this valuation,

2והעמידו THEN THEY SHALL PRESENT HIM — the person made the subject of a valuation, לפני הכהן BEFORE THE PRIEST, and the priest shall set a value on him according to the means of him who promised the valuation (Sifra, Bechukotai, Section 3 14).

3על פי אשר תשיג ACCORDING TO HIS ABILITY — i. e. according to what he possesses shall he (the priest) assess him (the person who has promised the valuation), leaving him sufficient to live upon: a bed, mattress and cushion and tools necessary for his trade; thus, in respect to the latter, if he was an ass-driver (earning a living by carrying loads) he has to leave him his ass (Arakhin 23b).