I am not a legally minded person, but from a purely technical point of view I don't see what the problem is with a manufacturer fitting an overhauled part to a production machine.
After all, the overhaul procedure is in place to "renew" a part and as long as there are no life limiting items involved that would affect the available hours to the owner before the part was due for the next overhaul.
Parts that are already used, however and have less than the full time remaining less test flying etc would be extremely dodgy and would incur a legal action if it were my aircraft.