Costa (spoiler)

[mpnorman10]

I defeated Fuller and after defeating Costa he says he still isn't going to give the Don's money. Defeating him several times does nothing new.

I am relatively new and suspect I am doing something wrong.

[Smaloki]

He won't pay up. You have to defeat him and then take the money. Make sure to sheathe your weapon before trying to loot him though - if you left-click on unconscious NPCs with your weapon drawn, you will perform an execution move which in this case would not only result in the city guard attacking you, but also you failing the quest and a later side quest that involves Costa.