I struck a deal with a slaver vessel. I will spare them for a slave. How do I attack after a successful deal? They are bad after all.
If you end an encounter with a peaceful resolution, there’s no way to manually force an attack. The only option is to move on.
The Friendly Slaver event has a special “blue” option if you have a level 2 teleporter where you can get a crew member and initiate combat.
However, as I mentioned, there’s no way to choose a different outcome once you’ve completed the event.