Transfer case leakage is a common issue on the 2004 Santa Fe.  It's
possible that it's nothing more than fluid coming out of the breather,
but
it's also possible that the transfer case is leaking.  And most of the
leaks are expensive to fix.  Removing the transfer case is very labor
intensive.  

Since your opinion is from a mechanic who likely isn't that familiar with
Hyundai vehicles and was relayed through the salesperson, it's a little
suspect.  If you don't have an account at www.hmaservice.com (requires
Internet Explorer), register and get one (it's free).  There's a very
nice
TSB detailing all the different types of transfer case leakage and what's
required to repair them.  Some require replacing the transfer case, and
you definitely don't want to hear how much that will cost.