Sorry for the necro, hope this counts as 'continuing the discussion'. I just wanted to let any other searchers with this problem know: you're not out of luck on the sugar quest if you sold/lost the Fort Avernum sugar, at least in the full version of the Escape from the Pit remake. There is another bag in Swamp City, one of the northern slith forts. That's the fort where
Click to reveal..
you have the option of crushing the slith eggs.
I had given up on finishing that quest after reading this thread, so I was very excited to find it randomly. No idea whether it's there in the original or not, but the two games seem to be very similar so it may be worth a look.