Arachin 17 - When One Pays What He Can
When one vow to donate his symbolic value to the Temple, he pays according to the age and gender guidelines given in the Torah . If he is poor, he only pays what he can afford, as estimated by a Kohen.
What happens, however, when he promises the value of someone else? In this case, the "what he can pay" dispensation applies to him, not to the one whose value he promised. Thus, if a poor man vowed the value of a rich man, he gives the value of a poor man. By contrast, with the sacrifices it is different: if a poor man vowed to buy the sacrifices for a spiritual leper, "metzora," and this leper is rich, the poor man has to pay for the sacrifices of a rich one. Why is this difference? The symbolic value obligation did not exist until he promised it, therefore, it is his obligation, and it depends on his wealth. By contrast, the obligations of the leper already existed: due to the leper's actions he got this spiritual condition, and it is his ability to pay that determines if he gets a discount.
Art: REMBRANDT Harmenszoon van Rijn - Parable Of The Rich Man