Formating invoice

I need to create an invoice for one client who has Several Clients each with a unique name, ID number, and Voucher numbers.

So I want the Custom fields to at the top before I do the billing. I should capture the custom fields for each Client

You can not within Manager.
You will have to use external software to either combine or divide the records you enter into Manager.

The solution discussed below would help you but I’m not aware of any intention of implementing it atm

You could try to use tracking codes but I do not believe they are really designed for your application.

Can you please explain your situation more clearly? If you have one customer, why are that customer’s clients involved? If you are only specifying that items on the invoice were associated with these derivative clients, that information belongs in the line item descriptions. From an accounting viewpoint, you have no relationship with your customer’s clients. And they should not be directly referenced in your sales invoice.

Placement of custom fields anywhere besides at the bottom of a transaction requires development of a custom theme. See this Guide: