Arachin, Chapter 4, Mishnah 1. Perek alef established that three factors set the sum owed on an erech vow: how old the person is, whether the person is male or female, and how much money is available to hand over. That leaves an obvious gap. Every erech vow has two parties standing in it: the noder, who speaks the words, and the ne'erach, whose worth is being pledged. Which of the two supplies each of those three measurements? As it turns out, they are divided between them: certain factors are read off the speaker, others off the person he spoke about. Our mishnah goes through them in order and assigns each one to its proper party.
"Hesseg yad banoder": the ability to pay is judged by the person taking the vow. When a person cannot cover the sum the Torah assigns, we look at his own net worth, his own financial standing, and not at the resources of the subject of his vow. The mishnah itself will return to this point at the end and spell it out.
"Vehashanim banidar": the age bracket, which determines into which of the Torah's sums the pledge falls, is taken from the subject of the vow, not from the one who made it. Take a man of thirty who declares the erech of a man of sixty five. He pays the figure the Torah sets for a man of that age, fifteen shekalim, and not the figure that would have applied to a thirty year old, which is fifty shekalim. The years that count are the years of the one being spoken about.
"Veha'arachin bane'erach": the male and female categories, which likewise shift the amount the Torah requires, are determined by the ne'erach. It is his or her status, the person whose worth was pledged, that fixes which column of the Torah's table applies, and the one who pronounced the words does not enter into it.
"Veha'erech bizman ha'erech": and when we fix that age, we take the subject as he was at the moment the vow was made, not as he is at the moment of payment. Suppose a person pledged the erech of someone who was nineteen years old, and never got around to paying. By the time he comes to hand over the money, that individual has turned twenty, and the Torah's sums for nineteen and for twenty are not the same. The vow is measured by the day it left his lips. The subject was nineteen then, so nineteen is what he pays, even though the man standing before him now is twenty.
How the Ability to Pay Works
The mishnah now elaborates on the first of the four rules. If a person cannot afford the full sum of his erech vow, he is assessed by the kohen, who examines his circumstances, and he pays what is within his reach.
"Hesseg yad banoder": financial capacity is measured off the person who pronounced the vow. "Keitzad?" The mishnah asks for an illustration and supplies one. "Ani shehe'erich et he'ashir": a pauper, who has nowhere near the full amount, pledged the value of a rich man, a man who could have handed over the entire sum without feeling it. "Noten erech ani": the obligation is a pauper's obligation, whatever the kohen determines to be within his reach.
"Ve'ashir shehe'erich et he'ani": now turn the case around, with a man of means pledging the value of a destitute man. "Noten erech ashir": the wealthy noder owes in full, the entire amount the Torah names for that age and sex, with nothing knocked off. Poverty of the ne'erach is simply irrelevant here, since the only pocket the halachah examines is the pocket of the one who accepted the obligation.