You can create your own variations on the standard invoice under the View Templates tab. Some knowledge of HTML is required, but you can easily find someone with those skills if you don’t have them. You could have a Cash Invoice version and a Credit Invoice version; Manager will allow you to select the proper form via a dropdown box when you create a new sales invoice.
It would be only slightly more difficult to add signatures boxes.
Manager already has this capability. See http://www.manager.io/guides/sales-quotes