Gemara

86b (Rav and Shmuel): If one sold 'a Kor for 30, each Se'ah for a Sela', after each Se'ah is measured out, neither can retract (the sale of that Se'ah).

87a - Question (Beraisa): One may not hire a worker for a Dinar per day of work to be done later, if the normal wage is a Sela per day (the discount looks like Ribis). If he hires him to begin now, and pays immediately, it is permitted, even though the normal wages at the end of the job will be more;

If when one sells 'a Kor for 30, each Se'ah for a Sela', (only) it is acquired (but either can retract about the rest), here also, at any time he need not finish the job. He works the last days for a lower wage because he was paid in advance. It should be forbidden (like in the Reisha)!

Answer (Rava): We are never concerned for one who works for a small wage!

In the Reisha, he is paid before he starts working, so it looks like Ribis. In the Seifa, he starts working immediately, so it does not look like Ribis.

Bava Metzia 65a (Mishnah): One may charge more rental if the renter pays later, but one may not charge more in a sale if the buyer pays later.

One may set the rental for his yard to be 10 Sela'im for the year if paid up front, or one Sela per month. One may not set the price for his field to be 1000 up front, or 1200 if paid after the harvest (when the buyer will have money).

Question: What is the difference between rental and a sale?

Answer (Rabah and Rav Yosef): Rental is owed only at the end of the term. The real rental is one per month, just Reuven gives a discount for paying early. In a purchase, the buyer owes the money immediately (the real price is 1000). Reuven charges extra for delaying payment.

(Rava): We learn that rental is owed only at the end of the term from "ki'Schir Shanah b'Shanah." Rental for this year is paid in another (i.e. the beginning of the next) year.

73a: People who guard standing crops used to be paid after the grain was harvested and threshed. They received extra due to the delay.

Rava: (This is Ribis.) You must help a bit at the granary, in order that your work continues until then, and there will be no delay in your wages. Wages are due to be paid only at the end of the job.

Rishonim

Rif and Rosh (Bava Metzia 36a and 5:21): A Tosefta says that if one sold something on condition that the buyer pay after a year, he may offer it for a lower price if he will pay immediately.

Hagahos Ashri: If rental would be due immediately, if would be forbidden to charge more for paying at the end, like in a sale. Therefore, one may not say 'I will allow you to pay at time Ploni (after rent was due) on condition that you pay extra', it is forbidden. Similarly, if a teacher was paid late, and he demands the profit that he could have made with the money in that time, the employer is exempt, for this is like Ribis.

Teshuvos Maimoniyos (Sefer Mishpatim 15): R. Eliezer of Tula (some texts - Tuch) said that he may demand the profit that he could have made. This is not Ribis. We learn from Makos 3a. If Edim Zomemim falsely obligated someone to pay early, they pay the amount that a person (Ploni) would pay to keep money longer. This must refer to wages Ploni owes, and we infer that Ribis does not apply to wages. If not, Ploni may not pay anything, due to Ribis! R. Eliezer's brother asked from Bava Metzia 73a, which implies that Ribis applies to wages.

Beis Yosef (YD Sof 160 DH ha'Me'akev): Why was R. Eliezer more lenient about wages? The general rule is, all Agar Natar (wages for delaying payment) is forbidden! There is no proof from Makos. We evaluate what one would pay, e.g. to a Nochri, or to a Yisrael who transgresses, to have the money for this time. The Tana did not explain this, for he was not teaching about Ribis. Surely, R. Eliezer's brother gave this rebuttal, but R. Eliezer did not accept it. Hagahos Maimoniyos did not need to bring it, for it is obvious. I do not know why R. Yerucham cites such a Heter in the name of Rabbeinu Meir without arguing. One may not rely on it.

Bach (161:2): Rabbeinu Meir supports R. Eliezer from Makos. Had the witnesses physically seized the money (for this time), they would be exempt due to Ribis. All the more so they are exempt for doing so through testimony! Rather, we must say that Ribis does not apply to wages. There is no proof from 73a. Presumably, the wages owed were immediately converted to a loan, lest the employer transgress not paying that day. We can dispel Hagahos Ashri's proof. Once he said 'if you pay now, it is yours for 10', it is as if he converted it to a loan. This answers all the Beis Yosef's questions. In practice, we leave the money where it is. If the worker seized it, we do not force him to return it. In this Siman the Tur says that Ribis does not apply to Sechirus, i.e. mid'Oraisa it applies only to loans.

Rambam (Hilchos Malveh 7:8): One may charge more rental for land (for paying late). One may rent his Chatzer 'for 10 for the year if you pay me now, or one per month if you pay each month.'

Hagahos Maimoniyos (4): R. Shimshon says that this is when he entered immediately. If not, it looks like Ribis, like we say at the end of this Perek.

Machaneh Efrayim (Dinei Ribis 31): The words of the Gemara connote that one may charge more rental (if he will pay every month) only if that is the proper value. However, based on the reasoning (the rental is due at the end), it should be permitted even if it is worth less. A support is that we forbid a similar case of a sale. This is only when it is worth less (Rambam 8:1, unlike Beis Yosef in the name of Mahariko).

Rambam (10): One may not charge more wages (for paying late).

Rambam (12): One may not hire a worker in winter to work for him in Kor** (the end of winter, when it is coldest) for a Dinar per day and he paid him (now), and his work in winter** is worth a Sela per day. It looks like he lends to him today in order that he will work for less money. If he hires him to work from today until day Ploni for a Dinar per day, even though his labor is worth a Sela per day, it is permitted. Since he begins now, it does not look like he receives (a discount) for paying his wages early.

Note: Some say that these are printing errors; both should say 'summer', like YD 176:8 (Shinuyei Nuscha'os in Frankel Rambam).

Magid Mishneh: The Rashba says 'my Rebbi (R. Yonah) says that we forbid only a worker. Since he does not begin now, it looks like Ribis, for he is not totally obligated; he may retract. Rental of land is totally acquired immediately, so one may rent it now for less even if later it is worth more.' I.e., we permit even if he does not begin to live in the Chatzer now.

Machaneh Efrayim (ibid.): The Rashba learns from Bava Basra that one may not work for less than normal when he is paid in advance. I say that this is only when he is hired by the day, for each day is a separate contract. If he hired him for a set time, it is permitted.

Rashbam (87a DH v'Tisbera): Often, if one has no food, he accepts to work for less than the normal wages. Therefore, it does not look like Ribis.

Poskim

Shulchan Aruch (YD 176:6): One may charge more rental for land. If one rented his Chatzer, and before the tenant made a Chazakah he said 'if you pay now, it is yours for 10 for the year. If you pay each month, it is one per month', it is permitted. The same applies to wages.

Rema: If he already made a Chazakah to rent for 10 for the year, he may not say 'pay one each month, and I will wait for the money.' All the more so, if he already owed the wages, he may not give extra for delaying payment.

Source (Beis Yosef DH Mutar): Talmidei ha'Rashba say that this is absolute Ribis. This is obvious. He does not mean Ribis Ketzuzah (contracted), for anything not through a loan is not Ribis Ketzuzah (Rambam Hilchos Malveh 8:1). Rather, it is proper Ribis, and not merely a ruse for Ribis. Or, since he already made a Chazakah in the Chatzer and acquired it for 10, and now he pays more because the payments are delayed, it is as if he gave to him the 10 and took it back, it is truly Ribis Ketzuzah.

Taz (8): When he already owed the wages, the Beis Yosef would agree that this is absolute Ribis. He was unsure when Chazakah was made, but the money was not yet due.

Shach (8): This shows that there is no difference between money owed for rental and a loan, like the Beis Yosef, and unlike the Bach.

Shulchan Aruch (8): If one hires a worker in winter to work for him in summer for a Dinar per day and he paid him (now), and his work in summer is worth a Sela per day, it is forbidden, for it looks like he lends to him today in order that he will work for less money. If he hires him to work from today until day Ploni for a Dinar per day, even though his labor is worth a Sela per day, it is permitted. Since he begins now, it does not look like he receives (a discount) for paying his wages early.