In base management - sends screen it's possible to add for sending more stuff, than you have. I don't know, does it really such transfer.
When all copies of some item kind are added to it, amount on sending base is shown = 0. Clicking on item name, instead of counter arrow, will add for sending more copies: first of such clicks changes from 0 to... nothing (sic, nothing displayed), and next clicks increase it into negative.
For example, we have 2 alien missile launchers on ufo disassembling base, and want to send it to production base. Say, we already added 2 items.
- Now click in item's caption in base content list, it will set amounts to sent=3, remaining="nothing displayed".
- Net click sets it to sent=4, remaining=-1
- next one - sent=5, remaining=-2
- and so on
As i said, i did not even try yet to do such send.
Searched for existing bugs and forum posts by keywords "send, negative, amount, count" - nothing found.