lordhoff: Just buy it back - so a little gold is lost :) Been there, done that with several items. I believe there is a way but I've never tried so really can't say. I think there are sites on the I-net that specialize in cheats for a particular game that would likely have the needed code.
rolgurg: I would but i can't find it :( i also sold it during the previous chapter so not sure if i can buy it back anyway :/
Edit: Just a question but if i lock Fernando up ( because of bandit's weapon dealer quest) will it affect the quest fernando's business ( is that quest really important ? )
You know, I'm not really sure (been a while) but in general, if it logically can't happen due to some action, it won't. A lot of the tasks yield a few this or thats - missing a few won't stop you from beating the game.
As for the cheats: try this page:
http://www.neoseeker.com/Games/cheats/PC/gothic2.html It seems pretty complete. I did a search with "Gothic 2" and "cheats" and a bunch came up. The actual gaming sites should be safe enough and I've been to "neoseeker" before without any ill affects (no promises, though).