There are 2 paths you can take when creating an Invoice in Productive.

The Billing > Invoices screen shows all of your Invoices that have been created on all projects. 

If you want to create a new Invoice from here, click on + Invoice and select the company for which you will be creating the invoice:

You will then be able to select which budget(s) you want to invoice:

Use filters to narrow your search:

You can create an Invoice for a specific project while on that project's dedicated page in Productive. 

You won't have to select a project after clicking the "+" button. Simply select a Budget and create the Invoice that way.

Productive allows for creating partial invoices by entering the percentage of the budget that will be invoiced:

For example, let's say we want to invoice 20% of this budget. This will affect the quantities of the budget:

Invoicing using budget percentage can be chosen both for using 'by service' and 'by budget' methods:

After the invoice has been created, the Invoicing tab will show that 20% has been invoiced and 80% is left for invoicing:

Now, we can easily create an invoice with the remaining 80% that is left for invoicing.

By entering the amount greater than what is left for invoicing, a message will appear below:

By clicking on the Show details button, you will see what is being over-invoiced along with the amounts next to the budget and services (or budget only if the method 'by budget' is selected).