How to do invoice partially by value (amount) in Invoicing?

How to do invoice partially by value (amount) in Invoicing?

 To do invoice partially, first create milestone line items in Sales order and invoice 1 line item at a time.

Eg.

1. Create a Quotation for 100k - Single line item
2. Create Sales order from quotation for 100k- Single Line item
3. When you want to invoice 20k, edit sales order and add one more line item no 2 as 20k, update line item 1 as 80k and save.
4. Now you can invoice the second line item alone and leave the balance 80k in first line.
5. You can also update the description accordingly.

This will give you better control as well. This is how we issue our invoices as well.