I have German OS X, so maybe some names are different.
1st you need a group for freelancers and your freelancer "f" in it.
Then in Server Admin -> Sharing you put this group in ACL from share A.
Choose "custom"-rights -> then expand the "read"-area and there you should see something called like "cross " or "transit folder". That's what your freelancers need -> accept and save.
Don't forget to transfer the rights to the subfolders.
Now you can make a new share with folder B and there you give your freelancer "f" access rights as you wish.
Users in the freelancer group should be able to see the share A and B but not to enter it.
Your special freelancer should see both shares, but only be able to enter share B.