Dear Abul Rahim,
few years ago there was a SAP consultant in our company and together with my colleague bulit this solution. It was never used (for down payment is still used very old solution which is hard to modify so I decided to try this "new one").
How it should work. Create standard sales order with requested materiál items. Then if down payment should happen, insert new service item with item category TAO. Then for this service item create FAZ invoice. After this document is cleared by incoming payment, delivery from sales order is created. And then standard F2 invoice.
This scenario works well, the only thing I do not like is to fill "Billed value" manually to "Billing plan".
As I assume, you mentioned use TAO item category for all "materiál" items. I am not sure whether this is proper solution. If I will have dozens of items to modify in case of down payment is necessary - sales people will hate me. And I am not sure how to handle delivery from this items. For me is preferred solution to use extra service item for down payment. If it means that there is necessary to fill amount, so be it. But if there is a way how to do it automatically, I would be happy.
Thank you and best regards,
David