Also stay in the loop with what's going on through these awesome links:
Now once invoice is created and partially paid you cannot change the figures billed to client. The problem is that, if for example, a client reduces the work to be done after making partial payment, the invoice was for the initial cannot be change. For example sent invoice to client for $13900, client paid $10,000k and decided to reduce amount of work to amount already paid. Now you cannot edit invoice to change the initial amount from $13,900 to $10k.
You say this is something that already exists, but it's June 2021, and it's still an issue.
You can not edit once a partial payment is made. That is the issue the original post wanted to address and we are having the same issue.
You have the ability to edit invoices until they are paid.