Your machinist buys you a new piston, machines it correctly, and gives it to you as compensation for his error, and his error alone.
Otherwise, you take your parts and don't pay him.
Isn't that the way things are done in Europe?
If this was in Germany, the machinist would have been apologising that the new replacement piston wasn't already there, and have refunded your money as a general principle for failing in the most basic of apprenticeship skills: Marking those bits with their completed machining numbers!
Mistakes will happen, there is ABSOLUTELY NO REASON that you should have to pay for it though.
He owes you a piston, machined properly.