Will the game actually allow you to pay that amount (and end up with more money than you did before), or does the game decide that you don't have enough, even though you clearly have more than -5 gold (barring hacking your gold count, of course)?

In any case, this reveals that the cost is represented as a signed integer. If it were represented as an unsigned integer, I would expect it to cost something like 251, 65,531, or even over 4 billion gold just to identify an item.

I can't check that in my conversation player. BTW Another thing. NPC's (or at least Dominus) will take more gold if you give them more than they require. If he wants 10, and you give him full stack (e.g. 20 gold), he'll take all. That I checked in game too, but I noticed it because there are no calls for functions that "return" some inventory to player, just "get" inventory from player.
Also, because of this testing, I found a bug in my code, so it was worth it. I'll release this full level - conversation - graphics editor soon. You can check some random tests I made couple of weeks ago here :
https://www.youtube.com/watch?v=tgT272P8AkU&t=1s