well.. you can climb back out of the fortress and go back snatch the potion. but you can't climb out when you're already inside the building
but if you're already stuck,
here are my saves:
http://www.sendspace.com/file/5kqtpr i played theif
backup your old ones just in case and then replace them with these
the savegame that says "i have dispell" is what you're looking for. or you don't have ingredients for dispell yet? in any case, you can check out the saves, there are some more
i think they are also ordered chronologically from the oldest to the newest bottom/top