Negotiations: Removing items

Hello,

I just negotiated with another player. He wanted 5 technologies
from me and made them show up in the list, but I thought that 3
technologies should be enough, so I wanted to remove 2 of them from
the list. But there is no button for removing items, so the only thing
we could do was to cancel the negotiation, start another one and
reenter the 3 technologies.

Is there a way to remove technologies/maps/gold etc. from the list?
Because there really should be one...

-- 
Best regards,
 Daniel