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.
No, if you choose to purchase from them you will not be able to attack the slaver vessel during the Friendly Slaver Vessel event.
If you want to obtain potentially 2 crew members from this event, you will need at least a rank 2 teleporter and pick the blue option in an attempt to free some slaves.