I think that its thoughtful and generous of you. However, it is a bodily fluid, and something that people would be giving to their small babies which is why the donation banks have such a rigorous process with screening the mom and the milk. I thought I was a pretty healthy living person, but after looking at the list, I realized that they would not take my extra milk. If you cant go through that process, I think it would be better to just throw it out.
I would hope that a mom would be leery of taking breastmilk from someone else and even though its good enough for your baby, and you THINK its perfectly fine, what if the other baby got sick from it, somehow? I dont know if its likely, but it would still be a concern.
Maybe look into some alternative uses of breastmilk - everything from using in a bath to treating a bee sting! http://bliss-breastfeeding.blogspot.com/2009/06/things-to...