Ran into the following bug on Android:
 
After shopping, our party had 7 gold left.
 
We travelled to a sidequest and a travel event hit us. We had the choice between having 16 gold stolen from us or doing something dangerous.
As we only had 7 gold, we chose to loose the money.
 
Now we have -9 gold.
 
Thievery shouldn't be able to instantly put you into debt.