@ecollot I have the same case here and I have a workaround solution (this is called progress invoice).
Please see the sample:
Now to do this, you will have to create custom fields for your invoice item (Qty, original amount, and billing %).
Now you will have 2 columns for qty, you will have to make the original qty hidden and the custom one printable.
Then, you need to fraction the qty on the original field (which hidden to client) and now you have progress invoice.
Regards,
Yahya - CFO