I was working on these ideas with the free version of fusioninvoices and would be glad to provide any assistance.
It would be good to have at some point the ability to create credit invoices in order to add Suppliers logic as well by supporting both accounting transactions.
The invoice_groups table will need an extra field, something like invoice_sign (-1,1) and that would be used in order to describe if this is as credit or debit type invoice group. This affects the whole system calculations, so some work must be done in general.
I don’t think that a new supplier table would be necessary, since a company (or individual) can be your client and your supplier at the same time.
Some times, it happens to get a project that needs to be partially outsourced. In addition to the above credit invoices logic, it would be great to have a way to visually group some supplier invoices with your regular clients invoice. For example, i get a project for developing a new application and i need to outsource the graphic design part and the content part as well. So when the job is done, i will issue an invoice to my client and will have 2 invoices issued to me from my suppliers. I would like to have a visual way to see that for this project, i’ve earned xx amount but paid yy and zz amounts.