How do I return a serialized item that was sold prior to RMS being installed?

You will need to use a non-serialized entry for this purpose then make adjustments to inventory later. Example: Create an item called "Prior SN Return" with prompt for price at POS. Make this item taxable to properly credit the full amount.

Later you will need to adjust the quantity for the returned item and increase the quantity for the serialized item, entering the serial number at the same time.

In effect you are receiving new inventory by processing a return at the counter. The item cannot be retuned under a serialized item entry since there is a record for that serial number.

Another way to handle this is to create a credit on account or issue a gift card in the amount of the return. Make a manual adjustment to the drawer to account for the dollar amount and later receive that item into inventory.

You could also add a reason code in Manager, Database, Reason Codes to track and manage these transactions. If the item is returned to main inventory it will need to be adjusted as above. If returned to offline inventory, you would handle it later as a Transfer Out/In. Transfer In will prompt for the SN when received.