What to do? Depends on what you want.

First off, I'd wait on contesting the charge until that's the last option
available. If all the other work is OK (and I'd be suspicious if I were you)
and they fix the cluster to your satisfaction then they deserve to get paid
for work done.

That said, I would not be satisfied with a used unit if the damaged one was
new. And I'd agree with you that if the mechanic damaged the unit once then
he could easily do other damage. The solution then would be for them to
credit you for the cost of the parts, plus labor to install it (good luck).

A good manager should want to keep your business and will try to work
something out to everyone's satisfaction. A poor manager might result in the
charge being contested and no-one on this list using that shop in the
future.

Let us know what happens,

john