After a bit of miscommunication, my wife and I ended up buying 6 litres of milk between us. I don't really want to waste it, so I'm wondering if it's safe to freeze some of it and defrost and use it later? 
Worked fine for us; we used to cottage with people who didn't want to shop very often, had lots of kids and a big freezer out back.  We would buy lots of milk and freeze it just fine.  Do let it defrost completely before trying to use it.  We never noticed any separation, nor problems with expansion - do be aware of the container the milk is actually in, and if there is room in there for freezing to occur.  FYI - this specific experience was in Canada, and milk up here comes in bags that we then place into a pitcher-like contraption to use.  Your specific container may or may not have enough space. 