If I loan a neighbor a bottle of milk, must I be concerned that when he returns another bottle of milk it may contain slightly more milk than I loaned him, and thus constitutes ribbis? (All bottles of milk will not necessarily contain precisely the same amount of milk even if it is the labeled as the same size.)
Chein: Some people worry about this. Others assume that people know the laws of Ribbis and that you are estimating anyway and are Mochel any tiny extra that you receive or any tiny extra less that they give back. Some people still are Chosheish about this though. Both are valid approaches. No one in this forum can tell you either way Lema’aseh.
chein: the issue is not the discrepancy in the quantity of milk, it is that the value of a quart of milk fluctuates and you may be returning more value than you received. The Poskim call this malve sa’ah b’sa’ah. Under certain circumstances, likely to apply in this case, it is permitted.
Hello: Why would the quantity not be a factor, if it is (slightly) different? Sam seems to agree it is a consideration.
I have a Talmid Chochom and Magid Shiur neighbor who borrows milk, and when returns it asks he we could be moichel if he takes a drop off so that it isn’t ribbis. (He is a tremendous anav and you would never otherwise know his hanhagos.) He does this even when the price of milk hasn’t fluctuated.
Chein: Hello is completely correct in theory. The only thing that matters is the value of the borrowed item. In most places nowadays, the price of something like milk will not fluctuate, at least not within a short period of time. So then the only concern is the amount that you give back.