In our Bava Basra Shiur (104B), we were discussing the concept of when a Sochar has to pay. The Gmorra quotes a Mishna which discusses a person who rents out a bathhouse and uses what could be two different terms; 12 Dinarim per year, one Dinar per month. So the question is does the renter get a free month in the case of a leap year because it could be construed that the rent is 12 Dinarim for a year (12 or 13 months) or it could be construed to be a Dinar per month so that the renter has to pay 13 Dinarim for a leap year.
This led to a discussion of whether the renter is paying monthly and whether he is paying at the beginning of the month or at the end of the month and whether paying in advance constituted Ribis in that the owner is getting his money "early". A cursory look didn't yield any information on the "time value of money" constituting Ribis; only over payment, or some other gift which could be possibly construed as Ribis. Are there any sources on the "time value of money"?
A tremendous Y'yasher Koach for your harbatzas Torah. I enjoy the insights and the questions/answers tremendously!
Your question is discussed by the Mishnah in Bava Metzia 65a which states:
"One may increase the rent but one may not increase the price of the sale. How is this? If he rented out his yard and said 'If you pay me now you can have it for 10 selaim for a year but if you pay monthly it will cost a sela per month' this is permitted. However if he sold his field and said 'If you pay now it costs 1,000 zuz but if you pay later, at the time when the wheat is threshed [this is a time when people have more money available because they have just sold their crops] it will cost you 1,200 zuz', this is forbidden."
The Gemara there explains that the difference between the Din concerning renting and the Din concerning selling is because "Sechirus Eina Mishtalemes Ela b'Sof" - one is only obligated to pay the rent at the end. (This answers one of your questions). Therefore when the owner made the offer on his yard at the beginning of the year, he was doing so before his client had any obligation to pay him. When he offered to rent it for 10 this means that he was simply giving him a discount.
In contrast, in the case of the sale, once the buyer enters the field he is immediately obligated to pay for it. Therefore since the seller offered to sell for 1,000 if paid now this means the real price is 1,000. Consequently when he added that if he pays later it will be 1,200 this effectively means that he is taking the extra 200 as a reward for waiting for the payment.
This of course is the principle behind the prohibition of taking Ribis - receiving more money in return for allowing the borrower to gain time. See the Gemara Bava Metzia 63b where Rav Nachman stated "The rule of Ribis is that every 'Agar Nater' is prohibited". Rashi DH AGER explains that if the seller takes a cheaper price because the buyer gives the money now even though the article being sold is not avilable, this is Ribis.