When a client signs up to my services, sometimes a sales order is raised to itemise the work and hours for the project. I then partially invoice the Sales Order until it is complete. 

This doesn't really work at the moment in Zoho Projects and I need to check how many hours were worked and then create an invoice from the Sales Order, update the quantity to the number of hours worked. Then update the timelog entries as invoiced.

Project invoices do have an Order Number field which is for the Sales Order reference, but this doesn't connect anywhere, it's just free text. I've tested various ways of raising invoices from projects to see if I can get the invoice to link to the Sale Order automatically but this doesn't seem possible.

I guess this could work if you could tag tasks in the project with items in the Items database. This way they could get added to the invoice as the correct Item and then linked to a Sales Order.

I would be great if the Invoice in Projects could detect if a Sales Order existed for the customer so you could select it rather than having to know the number and type free text. Or if you could add related Sales Orders somewhere in the project settings, so the correct ones would always be selected on the Invoice.