Adapted from the writings of Dayan Yitzhak Grossman December 3, 2020 All’s well that ends…
People will encounter many common ribbis problems without ever realizing that they exist. Some may be questions of ribbis deOraisa which must be returned. Other times they can be derabbanan which one should be machmir (stringent) to return. In all cases they must be avoided. Chazal are very stringent about any form of ribbis, and elaborate on the punishment, chas veshalom, of one who is lax in this mitzvah.
It is a lot easier to achieve the status of a “borrower” than one may think. Aside from actual loans, owing money to a friend for minutes used on his phone or for soda at the canteen, can also create a lender-borrower relationship. Having someone buy light bulbs from the store on your behalf and telling him to keep the change when you repay him, may be a ribbis issue. Hence these halachos can apply more frequently than expected.