I'm stuck in at this point (see the attached image): I can't do anything but go back down the pipe. If I ask Rufus to walk to the barrel (or anywhere else around), he moves his legs as if walking but he's actually stuck in the pipe, as if something was blocking his way. I suppose it's a bug or something?
Could somebody help me please? thanks!

Hmmm, if I remember correctly, you should be able to to talk to green Rufus in the sewers and exchange items with him. Yeah, it's probably a glitch. You can look for a collection of savegames on the developer's website, if your last manual save is too long ago.

I have already exchanged items with green Rufus... Anyway, thanks a lot for your answer! :)