Unless they can prove that Mr A was working for 
Cel Dealer B.  If he was (and I'm not saying this 
is the case with Mandarake) then I'm sure Studio 
Z could sue Cel Dealer B.  They may not be able 
to get the cels back form the Cel Collector but 
they probably could sue Mr A and Cel Dealer B if 
they were working together.  Might be tough to 
prove but if we're talking about that many 
million of dollar then....