Most likely an expendable part (union or seal plate) and the Delta Engineer thought, as most station guys do, that he was doing a good turn but somebody got their kni*kers in a twist over not being consulted 1st. Now you would expect that Delta will have to recertify the part before it can be used by them! However most IATA pool spares would be rotables and cover only AOG situations where the MEL does not give dispatch relief. An hydraulic repair kit might be a pool item and example, a part from it, if EK was not in that pool, then Delta could not supply it, as it would contravene the agreement with the other partners.