When a customer underpays an invoice by an amount, what can i do to show paid in full?

Question

If the payment is "close enough" that I don't want to re bill the customer, how is the best way to manage this in invoices?

UKR · Answer

From what I've learned about bookkeeping, once an invoice has been recorded, it can no longer be changed. 
If you want to forgive the amount due, post a New Credit transaction in your Customer Invoices account and categorize it to something like Discounts. This will not immediately mark the invoice as paid, but your reports or customer statements will show 0.00 due. 
To mark past invoices as paid create a 0.00 New Customer Payment transaction, click the "Apply existing credits" box and select the outstanding invoices from the list, to apply the credit amount. When you save this transaction, you will not see a new 0.00 transaction in your register but the selected invoices will have been mark as paid ("c" in the register's Clr column)

Greg_the_Geek · Answer

Just change the invoice to match the payment.