I assume you set up a notification telling you when cash is low.
At -500B, nothing happens.
If at war, and you have +5T, also nothing happens.
I understand the point but these situations never happen to players without money.
The serious cases I see, are empires with money sitting in other countries and on the account.
It is their choice to play in this way.
It is accepted.
but there are consequences to the lack of cash in a country.
One of the consequences is that you cannot purchase anything.
Another is, that your country makes the best effort to fix it because it gets to an absurd situation.
Take a loan if you wish but you cannot walk around with a negative amount of cash in you wallet and negative number of vegetables in the country stock.