Peah, Chapter 5, Mishnah 4. This mishnah deals with the concept of a temporary poor person: a homeowner who is traveling from place to place. "Homeowner" is the term the mishnah uses to describe a wealthy man, someone who is not poor. This man is on the road and has no money with him.
The words of the mishnah and their explanation:
"Ba'al habayis shehayah over mimakom lemakom" - a wealthy man who set out on a journey, and at that moment has neither money nor food with him.
"Vetzarich litol leket shichechah upe'ah uma'aser ani" - the gifts to the poor that are available to him: leket, shichechah and pe'ah, and also ma'aser ani, the ten percent of the produce that is given to the poor.
"Yitol" - he is permitted to take from them.
"Ucheshyachzor leveiso yeshalem" - once he returns home and stands again in his wealth, he must pay back what he took.
"Divrei Rabbi Eliezer" - this is the opinion of Rabbi Eliezer, that the temporary poor person is obligated to return what he took.
The Yerushalmi explains that the payment is made to the poor of the city from which he took, and not to the poor of his own city, since he did not take from them but from the poor of that place.
The opinion of the Sages:
The Sages disagree and say: "Ani hayah be'osah sha'ah" - we do not take into account that his money is sitting at home, since at the moment he took he had nothing with him, and he is considered poor in every respect. For this reason he is not required to return anything, since he was fully entitled to it at that time.
The halachah:
The view of the Rambam: although the halachah is decided like the Sages, that "he was poor at that moment," it is nevertheless a measure of piety to pay it back. One who wishes to be more exacting with himself should pay, even though the halachah does not obligate him to do so.
The view of the Vilna Gaon: there is not even a measure of piety here. The halachah is absolute: he was poor, his taking was entirely justified, and there is no value, and not even any preference, in his paying it back.
In summary: in this mishnah we learned the law of the temporary poor person - a homeowner who finds himself on the road with no money in hand, who is permitted to take leket, shichechah, pe'ah and ma'aser ani. According to Rabbi Eliezer he must pay when he returns home (and according to the Yerushalmi, to the poor of the place from which he took), while the Sages rule that "he was poor at that moment" and he has no obligation to repay. As for the halachah, the Rambam holds that it is a measure of piety to repay, while the Vilna Gaon rejects even that.