Well... the only thing you could have done differently would have been to copy the serial number down of the one that you received... I'm sure you didn't do that because why would you really?
I agree he's jerking you around, but there isn't much that can be done about it. Best thing to do would be just to be as nice as you can because he has your money AND your watch right now.
Is he an authorized Hamilton repair facility? If he isn't as soon as he opens the case he will void the Hamilton warranty... Might want to keep that in mind as you are bargaining with him. Make sure that if he is going to "work" on it you want written proof from him that he will essentially take over all warranty work on the watch during the term originally listed from Hamilton.
And don't freak out if he isn't an authorized repair facility. it's an ETA movement... regardless of what's wrong with it, it can be fixed and parts will be readily available until long after you are gone. It's not like a BMW where you go in for an HPFP replacement and then your cruise control stops working.