By 'the agent' you mean a DL agent? You should have never mentioned anything to the DL agent, just waited to put your AA number on the American ticket once it was issued, otherwise Delta would have had no way to every know you credited it to American. Even now if you simply ask for original routing credit because you were put on American Airlines I think you will likely have success, no need to get into the details of the frequent flyer numbers. I don't believe that ticket would have ever posted correctly to Delta without manual intervention after the fact.