AR will prompt you for the accounting date to use for the adjustment in AR, Tasks, Adjust Receiables.  You can unapply the cash receipt using a current period accounting date. This will change the cash receipt to a customer cash receipt that would then be available to apply to other invoices or to be refunded.  I recommend that you finish out of Adjust receivables after unapplying the cash receipt, then start this same task a second time to void the invoice. You can enter the accounting date you want the entries created as the result of the void to hit.  If you want it to be reversed out of a closed accounting period the process will vary.   You can void an invoice to a prior year once the year is closed as long as you haven't finalized the Financial statements for that year, but it may be best to wait to close the year in GL until after you have made the adjustment if you want the reversal to hit the prior year so that you do not have to reclose the GL (due to the prior year adjustments)

Thanks.