I have a P89 it fired more rds through it than I can remember. Hard to believe that it was working that well then just taking off the slide did that much damage.
My guess is that the gunsmith or whoever didn't push down the ejector when trying to remove and tried to push it down after the fact and broke it off. Just a quess.
You might try another gunsmith or looking for the parts on GB.
That is if the seller will not return your money.
Good luck and keep us posted.