Gemara

Tana d'Vei Eliyahu: Even though R. Akiva taught that it is better to conduct on Shabbos like on a weekday than to take from others, even an Oni should prepare something small to honor Shabbos.

Question: What is this small matter?

Answer (Rav Papa): It is Kasa d'Harsena (fish fried in its own oil with flour):

Shabbos 118a - Mishnah: If one has 14 meals, he should not take from the Tzedakah box.

Question: All agree that one eats at least three meals on Shabbos (and two every other day, a total of 15 in a week)!

Answer #1: It is like Chachamim, who say that one eats three meals on Shabbos; he does not eat on Motza'ei Shabbos.

Answer #2: It is like R. Akiva, who says that one should make Shabbos Chol (and eat only two meals) rather than to take from others.

Beitzah 15b - R. Yochanan: "Chedvas Hash-m Hi Ma'uzchem" - Hash-m tells Yisrael to borrow to sanctify Yom Tov, and to trust in Him to pay the debt.

Rav Tachlifa - Beraisa: A person's income is fixed, excluding what he spends for Shabbos and Yom Tov and to teach Torah to his children.

Question: Elsewhere, Chachamim taught that it is better to conduct on Shabbos like on a weekday than to take from others!

Answer (Hagahos Ashri 2:4 and Tosfos 15b DH Levu, according to Gra's text): If one cannot borrow, he should make Shabbos Chol rather than take Tzedakah. If he can borrow, he should, and Hash-m will repay.

Rishonim

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

Rambam (Hilchos Shabbos 30:7): One should prepare nice foods for Shabbos according to his means. If one can afford only cooked vegetables, this suffices. Chachamim taught that one should make Shabbos like a weekday rather than to take from others.

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.

Magen Avraham (1): One should have at least two cooked foods.

Kaf ha'Chayim (9): This refers to one who has one cooked food with his bread during the week. One must have more variety on Shabbos than he has on Chol. A second extra food should be added when Shabbos is Rosh Chodesh.

Magen Avraham (1): One should eat fish at every meal, if he enjoys it. If Nochrim raise the price of fish, it is proper to enact not to buy fish; we learn from Kerisus 8a. (The price of birds for Korbanos was exorbitant. To fix this, R. Shimon ben Gamliel taught that a woman obligated to bring many birds on account of births or Zivah can bring one bird and eat Kodshim, and need not bring the rest.)

Rebuttal (Chasam Sofer): There is different! R. Shimon ben Gamliel only exempted women from bringing all the birds, he did not forbid them to do so. Presumably, after the price dropped they brought the rest. Here, one cannot compensate after Shabbos! Also, there the concern was lest people transgress Kares (eat Kodshim b'Isur); here, an Oni who cannot afford fish is exempt! Nevertheless, it is proper to enact here to thwart people who jack up prices. However, it is not clear if this applies in other cases.

Shulchan Aruch (ibid.): 'Make Shabbos Chol rather than take Tzedakah' applies only if one is very pressed.

Magen Avraham (2): 'Very pressed' means that he has only enough for two meals each day. In such a case, he does the same on Shabbos and does not take Tzedakah. However, the Rambam (Pirush ha'Mishnayos Pe'ah 8:6) says that one eats three meals on Shabbos and does not eat on Motza'ei Shabbos, like the first answer on Shabbos 118a. The Rambam agrees that one who cannot obtain three meals without taking Tzedakah should eat only two.

Levushei Serad (2): The Rambam agrees only if one will have money to eat on Motza'ei Shabbos. However, if in any case he will need to take Tzedakah to eat on Motza'ei Shabbos, it is better to take on Erev Shabbos and have three Shabbos meals.

Kaf ha'Chayim (16): Those who distribute Tzedakah should give Aniyim enough for three meals on Shabbos. One should strive to have a fourth meal on Motza'ei Shabbos.

Taz (1): There are three levels. One who has money should economize on weekdays to avoid becoming poor, but he should not economize regarding Shabbos. If one is partially dependent on others, he honors Shabbos modestly, i.e. three meals and Kasa d'Harsena. If one is totally dependent on others, he is exempt from honoring Shabbos, and eats two meals like on Chol.

Eshel Avraham (2 DH Lovin): The Yerushalmi says that we (may) borrow on interest to honor Shabbos, i.e. in a permitted way (from a Nochri, or from a Yisrael through Heter Iska - Mishnah Berurah 4). The Aseh to honor Shabbos does not override the Lav of Ribis, for the Aseh is not fulfilled until after transgressing; also, the lender transgresses without any Mitzvah.

Rebuttal (Chachmas Shlomo): The Yerushalmi would not need to permit borrowing in a permitted way. Surely, it does not require an Oni who is exempt from honoring Shabbos to borrow on interest! Rather, the Aseh to honor Shabbos overrides Ribis mid'Rabanan, perhaps even Ribis mid'Oraisa.

Kaf ha'Chayim (17): If one owes a worker, he may not withhold the wages in order to honor Shabbos. He must pay him, and afterwards may request a loan.

Shulchan Aruch (ibid.): 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 his children Torah. Regarding these, whatever he spends will be returned to him.