What is the connection between the previous Parshiyos and the Parshah of Ribis?
Seforno: Having warned against the things that cause the Shechinah to depart from Yisrael, the Torah now issues various commands concerning Chesed that bring the Shechinah down to Yisrael, such as not taking Ribis 1 and Nedarim 2 - which includes the giving of Tzedakah. 3
Ba'al ha'Turim (in Pasuk 19) #1: Ribis is placed next to "le'Chol Neder" to teach us that one may not lend to take Ribis even in order to fulfill his Neder.
Ba'al ha'Turim (in Pasuk 19) #2: Ribis is placed next to "Gam Sheneihem" to teach that both the lender and the borrower transgress.
Seforno: Which one is obligated to give immediately following the Neder - since Aniyim are common.
Seforno: Also the Parshiyos of letting laborers eat [while working in the field]; not divorcing one's wife - if not due to immorality, in order to prevent Mamzerus; making one's newly-wed wife happy; not taking as collateral Keilim used to make food; guarding against people who cause public damage - people who kidnap small children, and guarding against the contagion of Tzara'as ? by isolating Metzora'im; guarding against tale-bearing and slander; the Chesed to a borrower - by returning his security, paying a hired laborer on the same day, leaving gifts for the poor during the ingathering of one's fruit; having compassion on a person who deserves Malkos and [not muzzling] an animal that is threshing, and on a deceased person who dies without children (to perform Yibum with her).
Having taught the sin of Ribis in Vayikra (25:36- 37), why does the Torah see fit to repeat it here?
Rashi and Ramban: Because, whereas in Kedoshim, the Torah prohibits the lender from lending on interest, "Lo Sashich" here implies a La'av on the borrower 1 , forbiding him to pay interest to the lender. 2
Rashbam: According to the simple meaning, this Pasuk is simply adding another La'av against a lender taking interest.
Bava Metzi'a, 75b: Who also transgresses "u'le'Achicha Lo Sashich" - in the next Pasuk - and "ve'Lifnei Iver Lo Siten Michshol" - in Kedoshism Vayikra, 19:14.
Ramban: As opposed to all other money-matters, where a person is permitted to cause himself a loss, if he so wishes.
Why does the Torah forbid the debtor to pay Ribis to the creditor?
Ramban: Due to the fact that Ribis is so commonly transgressed, the Torah includes the debtor in the Isur.
Due to the severity of taking Ribis - the transgressor forfeits his portion in the world to come- the Torah places a La'av on the debtor too - to dissuade him from encouraging the creditor to accept it.
Having said "Neshech Kesef", why does the Torah need to add "Neshech Ochel"?
Ramban: To teach us that, if Reuven lends Shimon a Se'ah of wheat for a Se'ah and a half, it is Ribis, even assuming that the Se'ah and a half that Shimon pays back is not worth as much as the Se'ah was worth when Reuven lent it to him.
What is "Neshech Kol Davar asher Yishach" coming to include?
Ramban: It extends the prohibition even to things other than money - with which one can purchase everything, and food - which constitutes a person's livelihood. 1
Bava Metzi'a, 75b: It extends the prohibition to Ribis Devarim. Consequently, if the debtor is not accustomed to greeting the creditor, 2 he is forbidden to greet him after receiving a loan from him (or by doing him a favor by enquiring after P'loni's wellbeing or whether so-and -so arrived safely on his behalf - Sifri).
But as far as other commodities are concerned, we would have thought that we go after their assessed value at the time of borrowing and at the time of payment [to permit even if he accepted (which is crucial to the Isur d'Oraysa - Bava Metzi'a 61b). (PF).
Otherwise, it iss simply an act of courtesy to do so - See Torah Temimah, note 105.