Gemara

(R. Yehudah b'Rebbi Shalom): Just like one's income is decreed on Rosh Hashanah for the year, his losses for the year are decreed. If he merits, he will give them for Tzedakah. If not, Romi will take them - "v'Aniyim Merudim."

R. Yochanan Ben Zakai saw in a dream that his nephews would lose 700 Dinarim in the year. He pressed them to give Tzedakah, and induced them to give all but 17 of the 700. The night before Yom Kipur, the Kaiser's officers came. R. Yochanan assured them that they will take only the last 17 Dinarim. He had not told them about the dream because he wanted them to give for the sake of the Mitzvah, without knowing that they were destined to lose the money anyway.

149a: Isar (a convert without heirs) had deposited 12,000 Zuz with Rava. Isar was about to die, and he wanted to give them to his (biological) son Rav Mari (who was conceived before Isar converted). Mari was away learning. Rava remarked that Isar has no way to give the money to Mari!

Question (Rav Ika brei d'Rav Ami): Isar can admit that the money belongs to Mari. Mari will acquire through Odisa (an admission, even if it is false)!

Word spread that Isar made an Odisa. Rava was upset that people counsel others in a way that causes a loss to Rava.

Beitzah 15b (R. Yochanan): "Chedvas Hash-m Hi Ma'uzchem" - Hash-m tells Yisrael 'borrow to honor Yom Tov, and rely on Me to pay the debt.'

(Rav Tachlifa - Beraisa): One's income is fixed, excluding what he spends for Shabbos and Yom Tov and his children's Talmud Torah. If he decreases, they decrease for him. If he increases, they increase for him.

Shabbos 118a: R. Akiva taught that one should make Shabbos Chol (and eat only two meals) rather than to take from others.

Rishonim

Rif and Rosh (Beitzah 8b and 2:4): "Chedvas Hash-m Hi Ma'uzchem" - borrow to honor Yom Tov, and Hash-m will pay the debt.

Rambam (Hilchos Shabbos 30:7): One need not pain himself to be Sho'el from others to honor Shabbos. Chachamim taught that one should make Shabbos like a weekday rather than to need to take from others.

Or Zaru'a (Drashah 20): One should not say 'I cannot spend to honor Shabbos', for it says "Lihyos ba'Shem Mivtachecha." However, one should not spend more than his ability, like we say 'make your Shabbos like a weekday, so you will not need to take from others.'

Me'iri (Beitzah 15b): Chachamim said, tongue in cheek, that one's income is fixed, excluding what he spends for Shabbos and Yom Tov. However, one should be careful not to need to take from others - 'make your Shabbos like a weekday, so you will not need to take from others.' However, for Kidush ha'Yom (at night), even one who cannot afford (wine for) it must strive according to his ability. Hash-m says 'borrow (relying) on Me for Kidush ha'Yom, and I will pay.'

Menoras ha'Ma'or 8 (Kavod Shabbosos): The Rosh expounded "Shalem (Shin Lamed Mem) Yeshalem" - if one spends excessively for Shabbos, Limud (learning Torah) and Mo'adim (festivals)", Hash-m pays him.

Poskim

Shulchan Aruch (OC 242): Even if one needs financial assistance, if he has some money of his own, he must strive to honor Shabbos.

Beis Yosef (DH Garsinan): One who honors Shabbos receives a boundless inheritance Midah k'Neged Midah, for spending without limit.

Aruch ha'Shulchan (44): We say that one should borrow and Hash-m will pay, i.e. if he has an income. Therefore, even if now he lacks, he can borrow and hope that Hash-m will enable him to profit and pay back. If one has no income, we say 'make your Shabbos like a weekday, and do not take from others.' The Rambam says 'one need not be Sho'el from others.' This can mean to request a gift; or, it means requesting a loan.'

Shulchan Aruch (ibid.): 'Make Shabbos Chol rather than take Tzedakah' applies only if one is very pressed. Therefore, one should minimize his weekday expenditures in order to honor Shabbos.

Mishnah Berurah (4): One's income is fixed, excluding what he spends for Seudos of Shabbos, Yom Tov and Mitzvos, or to teach to his children Torah. Regarding these, whatever he spends will be returned to him.

Chazon Ish (CM 21 Likutim for Daf 149): Why did Rava complain that people's counsel causes a loss to him? Income is fixed from Rosh Hashanah, just one must engage in wordly matters (Hishtadlus). A Chacham should not seek to cause a loss to Rav Mari! Granted, had Isar died (without an Odisa), Rava would not have been obligated to give the money to Mari. If he gave it, we do not say that Chachamim are pleased with him (Tosfos DH Magmerei). Still, why was he upset? Perhaps he wanted to use the money for communal needs; he followed Rav Huna's practice of fixing flimsy walls of poor people, and buying vegetables that vendors did not sell before Shabbos (Ta'anis 20b). Rav Mari would not have objected, since the money went for Tzedakah and Chesed, and Rav Mari himself was rich.

Beis Lechem Yehudah (on Rema YD 249:1): Calculation of Ma'aser is from Rosh Hashanah to Rosh Hashanah. One cannot deduct a loss in one year from earnings in another year.

Chavos Ya'ir (224): It is proper to calculate on Erev Rosh Hashanah, for this is when income is decreed.

Ha'Rif (in EIn Yakov Bava Basra 10a Mezonosav): Hash-m decrees how much one will lose, but not in which way he will lose it. This is why R. Yochanan pressured his nephews to give Tzedakah!

Anaf Yosef (ibid., DH Darish): Beitzah 15b expounds that one's income is fixed. We learn that losses are fixed from the Yalkut (Behar), which expounds "Paros la'Ra'ev Lachmecha", that your food is Parus (cut), i.e. already from Rosh Hashanah it is decreed how much he will earn and lose.

Beis ha'Levi (on Torah, Toldos DH v'Hinei): Yitzchak blessed Esav "mi'Shmanei ha'Aretz Yihyeh Moshavecha." This is like he blessed Yakov! However, to Yakov he said "Hash-m will give to you." This is the real Berachah. He told Esav that he will receive from Yakov (when Yakov does not merit to give Tzedakah, it goes to Resha'im). This was a prophecy, not a Berachah!

Minchas Yitzchak (4:75 (3) DH v'Hinei): A craftsman is allowed to stop others from offering the same service in his city, lest they decrease his income. Really, they cannot decrease it, for it was decreed on Rosh Hashanah! Rather, when the Halachah allows him to prevent others, this is his Hishtadlus, and it is considered damage for the other to compete. When he cannot prevent others, he need not do such Hishtadlus, and the other is not considered a damager.

Ateres Paz (1 OC 6 DH v'Chen): Food for days other than Shabbos is fixed, i.e. dependent on Mazel. Food for Shabbos is from Hash-m. Shamai considered himself to be a Beinoni, therefore, he designated the food for Shabbos, until he found a better food for Shabbos, so what he ate (even during the week) was not from Mazel, rather, from Hash-m. Hillel served Lishmah, which is above Mazel, so he did not need to do so. Oters must do like Shamai.

Shulchan Aruch (419): It is a Mitzvah to increase the Seudah of Rosh Chodesh.

Tur: It is brought in Pesikta (of Rav Kahana 28) that one's income is fixed, excluding what he spends for Seudos of Shabbos, Yom Tov, Rosh Chodesh, chhm, and what one's children bring to their Rebbi's house.

Beis Yosef (DH v'Isa): 'What one's children bring to their Rebbi's house' refers to his wages for teaching them, like it says in Beitzah. Our Gemara does not mention expenditures for Rosh Chodesh. We can say that it is included in Yom Tov.

Rebuttal (Prishah 1): 'What one's children bring to their Rebbi's house' does not connote wages. Rather, it is gifts and food. Everything that the Pesikta discusses refers to food.

Bach (Sof ha'Siman): The Pesikta is a support for the custom that children bring money to their Rebbeyim on Rosh Chodesh. One should not annul the custom. It is the same whether one does much or a little (as long as he intends l'Shem Shamayim).