Yikes! Get a new mechanic pronto. This looks like the oil fill plug on top of the transmission case - the normal place to add transmission fluid. Your mechanic probably did not install a new metal washer gasket when he reinstalled the plug after filling. If the case is cracked, it may be because he over torqued the fill plug. If that is the case, you have a serious transmission issue that may require a new transmission to resolve. But that should be at his cost since he screwed it up. Lawsuit? It was stupid of him to apply putty to try to stop the leak.