Could we be Sued?

[Wiffle01]

Long story short:

Husband and Father-in-Law brought their classic car to a mechanic, paid $3000 up front, to have a then engine and transmission rebuilt. Mechanic was up front about the fact that he would be sub contracting out the work.

Time passed, more "problems" were found, price went up. now $6300 and "everything" will be done. Father-in-law paid out another $1000.

Now, Mechanic says he had a falling out with the sub-contractors. transmisson and engine still have problems. he wants to statrt from scratch. Says he won't charge anymore than the $6300 previously agreed on, but he will have to do the work in his down time, so it will take a while.

Husband asked me to research the legal implications of:
A) taking the car back and not paying the remaining $ (can we be sued for not paying)
B) taking the car back and suing the Mechanic for all or part of the $4000 already paid (is it worth it for time/effort/cost, and liklihood we'd be successful?)
C) Paying (because both husband and father-in-law hate confrontation), taking the car back and then suing because work was not done to standard.

Any advice is appreciated. Let me know if more info is needed. We live in Massachusetts, if that makes a difference.

Thanks in advance!