Once you have closed an invoice, you can manually mark that invoice as uncollectible. Marking an invoice as uncollectible will prevent the system from attempting to pay the invoice in the future. It will effectively “skip” this invoice, acting as if it were paid, and move to the next invoice. Here's how:
1) First, close the invoice.
2) On the invoice you wish to forgive, click the Mark Uncollectible button.