The bloodspot

 

Parshas Hashavua משפטים Mrs Borger borrowed a dozen eggs from her neighbour. Mrs. Borger may not pay her back 12 eggs. If Mrs. Borger borrowed a dozen eggs and found a blood spot in one of the eggs she is not required to pay back for the egg with the bloodspot. Since she is not required to pay for it if she does she will be עובר על איסור *רבית. Shulchan Aruch Choshen Mishpat  232:19 “If someone sells eggs to his friend and it was found to be spoiled and inedible, It is considered a מקח טעות,(mistaken sale) and the money must be returned. Today we don’t rule like this. And a Minhag is מבטל (nullifies) a Halacha” (The buyer is stuck with the spoiled eggs) Rav Y.S. Elyashiv & Rav S. Wosner zt"l both hold that this Minhag/Halacha is not applicable when a person borrows the eggs. Therefore there is no need to repay for the spoiled eggs.  R. Wosner (Shevet Halevi 10:133) writes that since there is no need to repay the eggs you may not repay it otherwise it's רבית*  דרבנן*