Excel Formula

I need some help with a formula i'm working on in excel (open office Calc).
I tried /g/ to no avail, can someone here help me?

Doing up a budget, and for the credit card balance, i'm wanting to make it so payments go towards it every week.
the general idea i have is if the balance is -$100 or lower, to put $100 in.
but if the balance is less than -$100 (eg. -87.63), for it to make the balance 0.

So far this is what I have:
=IF(PB<0;PB+(IF(PB<=-100;PB+100;???)~0))
PB = Previous Balance
??? = the bit i need

???=0 if I understand what you want to do correctly

should be something like:
=IF(PB<0;(IF(PB<=-100;PB+100;0)~0))

what's the "~0" meant to do?

that formula can't work in excel. :/