How can I make a decimal field default to a negative dollar amount?

I am doing a form that basically works like a check book.  I have one column that is ALWAYS going to be a negative dollar amount.  I've figured out how to make the number negative IF the user enters it with a minus sign (ex: -100 would show up as ($-100.00)).

However, I'm wondering if there is a way to get that field to be a negative dollar amount no matter what the user enters? Example: user simply types 100, when they tab out, I want it to automatically change to ($-100.00).

Is this possible?  Thanks in advance.