BSD Borrowing a (perishable) item in order to get it back (e.g. a bag of four for a bag of flour) may sometimes be a Ribbis issue. Why? Because the bag of flour may be worth $5 now but is likely to be worth more when returned, which is declared as Ribbis derabanan.
There are a few cases in which it is permitted to borrow in such a fashion. For example, one may borrow a small amount of food , as people are not particular about its monetary value and price change.