I should add that I agree with cathoo suggesting that no money exchange hands. Legal issues are not always the same as the way we see issues in daily life. If yarn was a dollar a ball and the day you purchased it was 95cents, technically that 5c is profit. Sure you can hand 5c back but just having someone else buy the yarn is cleaner and easier.
As RuthK suggests from her experience, designers are often ok but will ask for something to be done to give them credit (hence my suggestion re the tag).
Always ask and that way you avoid future headaches