The rules say that after an outdoor encounter, heroes may drink any number of potions and restore full fatigue. I don't see this rule for completing a dungeon?
Q: Under what circumstances can the heroes trade items,
drink potions, and recover fatigue?
A: In addition to the normal in-dungeon rules for trading,
drinking and resting, any time the hero party is on the
overland Terrinoth map, they may freely trade items and
drink any number of potions. They also are restored to full
fatigue as soon as they end any quest, be it dungeon or