The Don had already stolen the AD by the time you met him. Even if you never even talked to the Don and kill him when you first see him, he will still drop the AD. I believe that this is mainly in place so that you can finish the game regardless of the order in which you try to do things.
The whole story line with the Don and the AD just provides a way for you to acquire the AD without killing the Don. Something that I do not know is whether the AD will be for sale even if you have not talked to the Dark Savant in the swamp. I'll have to try that next time.
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