Qitzur Shulchan Arukh – 65:8
The borrower must be careful not to get benefit from the lender without his knowledge at any time when [the borrower] has his money. Even something he would have done even had he not lent to him. Since he received benefit without his permission, it looks as though he is relying on his forgiving him because of the money he’s holding. However, if he benefited with his knowledge, it is permisssible if it’s something he would do to him even if he hadn’t borrowed it — as long as it’s not something public.
Note that the prohibition here isn’t interest, but the appearance of interest. However, it would still seem that if he actually was relying on someone’s goodwill because they borrowed money fdrom him, it would be interest. (Since that’s the “interest” this case would look like.) Like the previous entry, there is some kind of linkage implied between this prohibition and assuming a tit-for-tat attitude toward doing favors.