It probably goes without saying that you can't ACTUALLY have -1 of something.  I'm referring to an event where a civ claims that pests from our civ has damaged one of their worlds, and they want 500 credits and 1 monsantium.  My issue is: first like I said, you can't have negative of an actual thing, but also, more importantly, wouldn't it be nice if you could put them on hold and sell some commodities or anomaly loot to raise the money?  On the same turn as the demand(s) come in, I could raise the money to avoid the diplomatic hit.  Instead if you don't have the cash that option is just grayed out.