Charges that appear on your account long after a purchase and secondary charges like tips can push your balance into the negative. Online retailers may put a temporary hold (charge) on your account when you place an order, then complete the charge when they ship your stuff. A restaurant may put a temporary hold on your account when you ask for the check, then process it later with the tip included. If there aren't enough funds in your Cash App to cover the late charge or the added tip, your balance could go into the negative.