@sansouci hey I think I could make a suggestion, your case is clearly more complex than the common issue with UGI merger that I discussed with @skhilko here; we both had the easiest task on our hands: complete disposition of all-long units. For the negative basis question I would ask one of the masters on this 14 page long chain How to record MLP sale I hope you can get helpful replies there or the answer may already be hidden in those pages, I vaguely remember seeing negative basis being discussed. Hope that's helpful!
