We don't know if the the mechanic ordered the wrong part in error or if the wrong part was delivered in error. Regardless, it's not uncommon for a mechanic to begin disassembly of a car prior to having all of the parts. It would also be completely reasonable to require payment for the additional labor of reassembly which would not be a part of the original scope.
By the way, performing work that is not in compliance of the NEC that is in force in the state/municipality can lead to fire and invalidate insurance in the event of fire. Also, scope documents should also be explicit in not only what is included but what is excluded and the general conditions that are expected and the assumptions that are made. If there are issues that are found to exist in an existing structure that do not comply with general assumptions, such s the building meeting code requirements for the time that it was built, for instance, and that requires a change order or an RFI then the customer may well be on the hook for that. Construction contracts are not absolute. The are simply an agreement as to what each parties expectations are. additional work requires additional payments. Reassembling a car, if it was disassembled per typical business practices, and you demand reassembly then that is a change in original scope and will require additional payments.