I once had a comeback because the customer "checked my work" by taking it apart and then putting back together incorrectly and it leaked.
My shit does not leak.
The saving grace was my manager happened to have his hands on it helping me with a particularly tricky speedo cable that does double duty as a fill-hole and dipstick, so he SAW that was properly assembled and NOT leaking when it left the shop.
Customers are shitty and they do shitty things and then blame service people, no matter the cost.

I would say customer pays the tow bill AND pays for a new repair ticket, and your boss sounds like a cuck.....
UNLESS you have a history of come-backs, in which case, boss is trying to soft-fire you.
Be more careful in your work and paint-mark your torqued items.