You can always save it, slip him the potion and see the scene, then reload it.
Save Scumming is helpful if you don't want to miss anything you might want to keep or see, you can do that before exploring the place where the enemy can drop your Sanity meter if you lose or submit to it, there are 4 ways to lose it currently, one of them might not be avaible on the Itch,io version at the moment until it get to version 12.0 in the future.