If it wasn't shemittah, you are right in thinking that returning more than the borrowed amount, if that addition is a recognizable amount, would be usury. This is true despite the fact that returning the same volume is allowed even if the product has increased in value, because it's assumed that such fluctuations aren't significant to neighbors or people who borrow small food items. (Mishna Berura O.C. 450:2 quoted in "The Laws of Ribbis" by R. Reisman, pg 269)
The laws of Shemitta do apply to lending objects (as in, where the deal was that the object would be replaced, not returned, which would be a case of borrowing) as it is no different than lending money. Thus, as long as the 'debt' was already due before shemittah (which, if not stipulated, is a period of 30 days) then shmitta would cancel that debt as well. ("The Laws of Ribbis" by R. Reisman, pg 359 quoting Chayim Shaal 2:38:13)
Returning money after shemittah can either be done as a repayment despite the shemittah, or as a gift of goodwill. If done as a gift, then as long as it's reasonable to look at the extra milk as such and not as reimbursement, then this would probably be allowed (as the Shach Y.D. 160:8-10 writes regarding a regular loan that wasn't cancelled by shemittah).
However, if the food is referred to as 'repayment', the question of whether or not extra food would be considered interest is probably dependent on a question of the Minchas Chinuch (mitzvah 477) as to whether or not shemittah cancels loans automatically, without any action or intent of the lender and borrower, or if it is a mitzvah incumbent upon the lender. According to the latter view, which believes that the loan itself isn't cancelled, it's likely that one would violate the prohibition of usury even after shemittah. (On this, see Minchas Asher Gittin siman 59- he doesn't mention this specific issue but he does discuss the meaning of shemitas kesafim in general at length).
Bottom line, I'd say (besides for saying to first CYLOR) that 'being neighborly' should be clarified that this is meant merely a gift, and not repayment, so that there's no problem.