Then they get zero income. And negative bonds. And probably an honour loss.
Seriously, the only scenario where this is possible is one where your total taxes exceed your income. How often does that happen? And if you managed to amass such a sum, shouldn't it be possible to actually put it away for a while?
Negative bonds aren't desirable to implement. How does it make sense for you to carry loads of gold, and a tax collector comes and says "well, okay, I see you have gold, but I don't like that, I like paper instead"?