Halakhah for Keritot 54:21

From the laws of the commandment is that each one of these four sins obligates its doer to bring a ewe or a female goat, like the well-known law of the fixed sin-offering; and he is only exempted with fowl or flour if he is poor. But if he is poor and he brings a ewe or a female goat, he has not fulfilled his obligation. And the reason is that since God, blessed be He, had mercy upon him and exempted him with [something less expensive] it is not appropriate that he push himself to bring more than what his hand can reach. And from this, every understanding person will acquire good counsel: to not make expenditures [that are] more than what is fitting according to his money - as this is a cause to steal from the creatures when he seeks that to which he is accustomed and does not find [it]. And they also said (Keritot 27b) that one who was wealthy and separated money to buy a ewe or a female goat with it and became poor and needs the money, should take two doves or two young pigeons; and he should say, "Behold this money is rendered profane upon these birds." And afterwards, he may benefit from all of the money. And so [too, if] he separated money for fowl and became poor and needed them, he renders them profane on a tenth of an eifah of flour, and benefits from [the money]. And so [too,] a poor person who separated money for a tenth of an eifah [of flour] and became wealthy, adds upon it and brings a ewe or a female goat. And for this matter, a wealthy person is called one so long as he has [the wealth]. And the liability for these sacrifices is only when inadvertent - and with an oath of testimony even when volitional - but if under duress, there is no liability for a sacrifice with any of them (Shevuot 31b). As the Torah exempts any [one under] duress from any liability (Pesachim 71b).