We were invoicing a customer that had a deposit linked to the sales order.  When we go to invoice the screen comes up to apply the deposit but when you type in the amount the screen disappears and won't let you continue to invoice for this one customer.  I also tried to bypass this by entering zero and it won't work.  I had to go in and unlink the deposit to the sales order and then go in after and apply the cash.  Why is this happening??