[GNC] Best way to close unpaid invoices?

[Daffy Duck]

Hello,

I have some clients that, for whatever reason, I didn't finish work for
because something happened, so we ended the transaction without the
full payment.

They continue popping up on invoice reminder, etc.

I have another client who I have agreed to settle for a lesser price
than that which was billed.

In these cases, how do "close" the invoice but still have the ability
to see the details if you ever have to?  What is the recommended way to
do this?

Thanks!