I just placed a few fish in the barrel (poisoned & void-tainted one time, and normal ones the second) and cast the aforementioned scroll on the barrel outside on the beach. Both times the fish were simply replaced with 1000 gold, and it seemed to have no effect on the Doctor himself when talking to him after, so I don't think you can poison him for the upper hand or anything, just a little bit of extra cash. lol
