Or is 10,000 the amount he is scripted to have after investing?
He's
supposed to get 500 like everyone else, but something is causing the script to run 20 times rather than once when he triggers it. I've thoroughly dug into the relevant script, the merchant quest*, and his dialogue tree, and can't see anything in any of them that would point to the problem.
*The game runs a quest in the background that handles things like investing, so that they only need one for all the merchants rather than one for each.