Sure,

Well first off.

IsNegative and Balance are idependantly mutable.

I could do this:

     var account = new Account();
     account.Balance = 500;
     account.IsNegative = true;

Or

    var account = new Account();
    account.Balance = 500;
    account.Credit(-600); 

Now Balance is - 100 and IsNegative is false.

Or vice versa for both.

I would expect anyone applying for a job that is asking for 5+ years of experience to pick up on something as basic as this.