There are some very subtle differences between voiding and refunding an invoice related to recording profit and cost averages.
A Void creates a 1:1 copy of the original invoice, with the same date, costs, and invoice information, but with negative quantities. A Void is the best option if a sale has been accidentally entered twice or if the sale never actually took place, as the transaction will be timestamped to match the date of the original invoice.
In most reporting, voids are recorded on the same date as the original invoice. The one caveat is that the inventory transaction log does not get recoded to the original date, so inventory movements will be reflected at the time the void is performed, not the time of the original invoice.
A Refund creates a reversal of the original of the invoice, with the same costs, and invoice information but the reversal is recorded on the date the refund is performed. This provides you with the assurance that you are reversing the invoice at the same price and the same cost that you sold it for.
In reporting, refunds are recorded on the date the refund is performed.