Invoices that show credit ballances

I am using ver 15.1.91 very successfully but would like to be able to have Invoices with credit ballances. This is because I often get a retainer or deposit before all the Invoice items are entered.

You should upgrade to the latest version which is 16.2. It has a concept of customer credits which can be used for payments in advance.

New feature noted by @lubos above replaces the previous response